Install a bidet.
Or just fuck up your toilet plumbing and tell everyone it’s a bidet. They won’t know the difference, and when everything goes comically, splashingly wrong they’ll assume they are at fault.
Damn, there’s a lot of good suggestions. I was just going to suggest shopping at IKEA, personally. 
Wine and pretentiousness were made for each other.
Always specify that your lead crystal glasses are handmade - not machine made - as though this may have been worrying your guests and you feel the need to put their minds at ease. Invest in sets of glasses specifically for certain varieties of wine - Merlot glasses, Syrah glasses, Pinot Noir glasses. Invent new types of glass if you need to - particularly effective if they appear superficially identical. Be patient, long-suffering and understanding as you correct your guests for matching the wrong combinations of glasses and wines.
Apologise for random things that you can blame on someone else. “I just want to say I’m so sorry about the green tablecloth when I’m serving veal! I know, I know, quelle disaster! Thom just wanted to help, bless his heart, and it was already on the table when I spotted it, and I didn’t want to embarrass the poor chap by pointing out the obvious. We’ll just have to make do.”
Of course, you didn’t actually go out and buy any of these exotic objets yourself; when questioned you must casually explain that, oh, yes, that clay raki decanter was shipped over to you from an acquaintance working in antiques in Turkey, oh yes, those dada prints, I simply haven’t been able to find anything of the sort domestically but my good friend Svorn picked those up while touring in Oslo…
This may be an urban legend, but apparently someone once asked Donald Sutherland why he starred in so many crappy movies, and he replied something to the effect of: " I have expensive taste in wine".
In the wine vein, it’s important you adopt the proper language which you can casually drop that would otherwise sound stupid, but now somehow has an air of sophistication. For instance, I was at my sister-in-law’s grandparents house for a dinner party (if you change this story, use first names only), and one of the guests when asked if she’d like her glass recharged responded, “Oh, I simply couldn’t - I’ve had elegant sufficiency.”
It’s curiously funny to me that serving things in hideous dishes is somehow pretentious. At any rate, last season Le Crueset had out some hideous cookware/serving dish combination. So, I got them for a friend who further took the theme and miss-matched the dishes to give it his own flair. One can’t go wrong with such.
Also, the Asterix suggestion is quite sharp I think, particularly if you keep around to remind you of “the language of maximum clarity.”
Always have on display a chess board with a game in progress, and some detailed hand-written notes nearby. Don’t allow anyone to touch the pieces. Say it’s a correspondence game with a French grand-master, and you think your new technical variation of the Sicilian-Najdorf will give you a marginal but promising advantage over her.
Hang some artworks by Teja Krasek on your wall, and pretend you understand the underlying mathematical concepts. For added effect, cite her published essays on higher-order symmetry. If you want to go the extra mile, casually mention that she is one of your favourite Slovenian artists (yeah, like you could name dozens).
Mask off a section of wall the area of a medium-sized painting. Lighten this area (using any suitable bleaching solution) so that it looks like the space left behind when a painting is taken down. Hide any evidence of your handiwork. When anyone asks, sigh in an exasperated way and say the insurance company insisted that you put the Picasso into secure storage. If your friend looks astonished, shrug and say, ‘It’s only a small one, and far from his best.’
Take any cheap, mass-produced fruit or herbal tea, and keep it in an antique silver box. Offer someone tea, and say you only ever use a very rare blend. Make up any name you want and say it comes from an obscure region of the world. Say that in keeping with ancient tradition, they only make 100 silver boxes of this tea per year, and that the boxes are exported all over the world. When someone remarks that you’re lucky to have got hold of one, smile indulgently and say that this is actually your fourth box this year, you simply love the stuff.
Buy a coffee table book of Lucian Freud paintings. Inside, tuck a small handwritten note on what looks like a cafe serviette saying something like, ‘With thanks always for your generous help, L.’ . Leave this lying around to be noticed.
Go all white…white carpet, white walls, white furniture, white cat…and remove all window coverings, even if you live cheek-and-jowl with your neighbors. Better yet, hardwood floors. Nothing soft or comfortable or useful in the place…no Kleenex box in the bathroom, no wastebaskets, no pillows except austere knife-edge squares.
Mr. Excellent’s and Green Bean’s Home Decorating Tips combined = Pretentious with a capital P…all barren black and white, with NO things you actually need or use. One could add a splash of color with an apple, red or green (replace apple as needed). In the kitchen: a drawer with a few takeout menues and a single plastic fork. Oh, and a bouquet of fresh flowers - every upper crust advertisement in magazines features a kitchen bigger than/more expensive than my house, with no actual signs of food or cooking, but there’s always a big honkin’ bouquet of flowers on the Ethan Allen table…The most in-your-face ‘machine for living’ I ever saw was Fay Dunaway’s pad in “The Eyes of Laura Mars” - grey flannel on EVERYTHING.
That doesn’t really strike me as pretentious. If I was going to do such a thing, it would be a decision more along the lines of “One is good, lots is better!”
Is midevil more pretentious than totally evil? I’m definitely sure it’s more pretentious than Resident Evil.
Which brings me to, either you have a good place to stash away your gaming stuff out of sight or you turn half the living room into The Games Room: a minimum of two consoles, 6’-tall speakers, etc. If you have the money to throw at them, there’s some computer brands out there which charge extra for pretentiousness.
Insist that you be adressed by your first initial and middle name. If you have the misfortune of having been saddled with a plebe-y first name (like Ray or John or Billy or something), make up a new one. Bonus points if you make people address you by your first initial and two middle names.
Just try saying this out loud: “So good to see you again, S. Farquhar! How were the Maldives?”
Yeah, that sounds more nerdy than pretentious to me.
I may be more pretentious than I realized!
Astérix books in French? Check. Novel by Spomenka Štimec, famous Croatian author, which I picked up in Helsinki because you can’t get it over here’? Check. Obscure Esperanto translation of Japanese manga? Check. Book in Punjabi that I did the cover art for? Check. Artwork by local artists? Check. Wall of white Ikea magazine holders? Check. Esoteric Polish bottled water? Check. Hand-made clothing from India? Check.
On the other hand, this apartment is cluttered and lived in, and is clearly a space where work is done. That’s not pretentious at all.
You really need to take it a step further. Bone up on their contents by reading literary criticisms online.
Also a copy of Winnie ille Pu. The *Tao of Pooh * is optional. I’d also recommend a copy of Green Eggs and Ham, because you need a few deliberate and well-chosen items around to show how unpretentious you are–which is really the ultimate in pretention. It could even be your pop of color in your otherwise ultra-neutral color scheme.
I vote against pure black-and-white, though. A little too '80s. Add in a palette of gray/grayed neutrals and you’ll be all set. Mushroom is a good color.
And the absinthe is a master stroke.
Heh. That’s what my apartment is furnished in. I love love love that style. My desk chair is actually featured in at least one book of notable chairs. My side chairs can be seen in the main guy’s office on Mad Men. I’m super-lucky. It’s all hand-me-downs. Which might actually be pretentious in and of itself. “Not only do I have good taste, but I come from a family with good taste.”
But my “wine cellar” consists mostly of Two-Buck Chuck. 
Buy everything Apple ever made, and talk about how swell it all is.
Fun 'n games with [old, castoff] architectural blueprints:
– Casually tack up on a study wall, because you just happen to admire them or were studying that style or architect, or find a detail interesting, or were contemplating commissioning the construction of a home or vacation home in a similar vein, or were considering hiring the same architect who drew this one…
– Elegantly frame as found art objects. I think this is actually less pretentious than the above, in which the prints are treated in a decidedly casual manner.
– Cover an undistinguished coffee table or dining tabletop, top with a sheet of protective glass, and voila!
– If you secure a large quantity of blueprint sheets, use them in combination with the “cover all your books with uniform white or brown paper” idea for your architectural/history/etc. library.
And let’s not neglect the role of pretentious music or ambient soundscapes… world music, hip fusion styles blending musical influences from First and Developing World traditions, pop music sung in French (I’m partial to Les Sans Culottes, who are [were?] Brooklyn-based Americans whose songs are seemingly inspired by cheesy vintage French pop music and sung en Francais), bootleg rock concert recordings, cast albums of Stephen Sondheim musicals…
Stock your entertainment center with foreign films (preferably with non-English titles) and express disgust whenever possible that they didn’t win Oscars.
No, no, to be completely pretentious, the Asterix books have to be in Latin or at least in a language you are completely unable to read. If you speak French and English, then the Asterix books must be in Japanese or Korean (Chinese is not pretentious enough), as you’d be able to at least guessread 40% or more of the Latin.
Oh, if you can get anything by Spanish group Héroes del Silencio or their singer, Enrique Bunbury, your pretentious status just skyrockets. Specially the group’s second album, which was so pretentious nobody understood it.
Stock a shelf with novels by Gabriel Garcia Marquez, Milan Kundera, Jose Saramago and the like. If someone admires your literary fiction, look slightly embarrassed, roll your eyes, wave your hand dismissively, and say “pfft! Beach reading.”
p.s. I love this thread.