This is gonna be a short OP because I only have a moment; I’ll expand later.
I’m going to be getting a new passport (my old one is several years expired). I’ve been thinking of changing my name for quite a while and been going by a slightly different first name (the one I want to change it to) for almost a year now. I’d like to officially change my last name, although my parents are rather upset about it.
So should I change my name now or wait until I’ve been going by the new name longer?
Why wait? You’re already using the new name. Worst case, you decide you made a mistake and have to change it back. Still easier than getting a passport changed or the hassle of having a passport with a different name then your airline ticket.
If you get your passport renewed and then change it, then you have to do the passport name change, which is a hassle. If you’re going to do it anyway, best to do the name change first.
If you’re definitely going to change it anyway, then might as well do it before you get the passport. Suppose it’s best to start as soon as possible getting identity documents in the new name, and a passport would make for one down already.
If you’re booking any travel soon, I’m not sure if there could be any potential hiccups if you’re paying for things on a card with a different name than your passport, but possibly not too much to worry about (and changing your name on a credit card could be the next step if you worried about that anyway I suppose).
Do it before getting your passport, definitely. It’s much, much easier that way.
And nothing forces you to actually tell your parents you’ve changed it officially. I changed my first name recently too, and am very glad I did (even though some of the change-over was a hassle).
It seems to me that if you are serious about changing your name, you might as well change it before the passport renewal because a passport is a serious document which states your name officially and which will not need renewing for ten years.
(I’d like to balance that notion with the idea of your parents becoming more accepting of your proposed name change over time, but since I don’t know if they will become accepting, that’s a really hard to balance factor).
I figure if you do change your name at all, do it before the passport renewal; you want to have all your docs in a row (sorry, I had to :p). You don’t want to have one name on the passport and one on your driver’s license.
There’s no requirement that the name on your airline ticket be your real name, only that it match the name on your identification. If you’re flying domestically, and you’ve already changed your name on your drivers license, just book your ticket with the name on your DL. If you then are flying internationally, book your tickets with the name on your passport.
As others have said, just make sure the name on your ticket is the same as the one on your passport.
Thanks everybody. Now I just have to find the money to do all this. 
If things go the way I want, I should be traveling next July or so, so I have time.
If you’re going to do the name change for a reason other than marriage, there are two different ways to go about it most jurisdictions, and it makes a difference on your passport process. I found this out the hard way.
You can either do a legal decree or a common law name change. In the legal decree, you petition the appropriate court, promise you’re not doing it with the intent to defraud anyone, etc. The judge issues you a decree and, often, you take out a notice in a major newspaper in the area. After that you change it with social security and motor vehicles (in that order).
The other way is a common law decree. You supply a form to the county recorder that you are changing your name, that it’s not for the purposes of fraud, etc., then lather rinse repeat with SS and DMV. This route is cheaper, since there is only the filing fee rather than court costs.
HOWEVER, the passport folks will immediately recognize the legal decree. You have to jump through a world of hoops for a common law decree. Under both systems, when you initially apply, you’ll have to provide the photos, your birth certificate and an official your change of name declaration. If you use the common law method, and haven’t been using the new name completely and officially for at least five years, your application will get rejected.
You then have to present two affidavits from people who have known you (IIRC) for at least two years and who have known you under both names, along with any supporting documentation (Bank statements/utility bills for the same account, with your old name before the change date/new name after, pay stubs with the old name and new name, etc.) and they will then review your case.
After a review of anywhere from 30 days to 180 days, if all your paperwork is in order, they will issue you a passport in your old name with a notation on the back page that you are also known as your new name. This will cause you modest, but not insurmountable delays when clearing customs or checking in for international flights. The agent will open your passport, see the name doesn’t match, look at you quizzically or like you’re the spawn of Usama bin Laden for a moment and then you’ll say, “Back page, please.” Every. Single. Time.
Why, yes, this is exactly what happened to me when I changed my first name five years ago. Why do you ask?
Wow - it’s so much easier when you’re changing your name by reason of marriage. The state department had my amended passport back to me in 3 weeks.
I did find when traveling that overseas customs/immigration people would automatically flip to the back page in my US passport when the name on my ticket/declaration form/whatever didn’t match the passport’s first page, but US immigration always seemed to have to think about it before flipping to the back. Unexpected.
Luckily I had to renew my passport last year so no more name mismatch for me.
Count me in the change-name-first-then-renew-passport camp.
Yep. Changing name for marriage is a common and uniform process, as is changing by court declaration. The common law name change, while every bit as legal, is not something the State Department protocols cover, since the process varies slightly in different jurisdictions. For that reason alone, if I had it to do over again, I’d have shelled out the extra $500 or so to get a judge’s nod.
Oh NOES, not again! However will I keep up?? 
When you get the time, I’m really interested in hearing more about your reasons for the name changes. You say you’ve been thinking about changing it for quite a while, I’d also be interested in knowing how long you’ve been considering it, and how old you are?
For me, I’d always hated my given first and middle names and planned to change them at some point. For years I went by my initials professionally and had everyone refer to my by my last name. I spent a few years kicking around names and combinations, found a first and middle that I liked and changed them as my 35th birthday present to myself.