I was reading a blurb about what guys want from women. One of the posters mentioned the adage “smart, attractive, sane – pick any two.” I googled it and came across the wikipedia article Project Triangle which is used in engineering and management.
A few variations are listed on the article but I’m wondering what others out there exist or that you’ve come up with.
We use this one all the time in market research:
You can have your study be fast, cheap, or accurate: pick two of the three.
For race cars or fast street cars: fast, cheap, or reliable. Pick 2 out of 3.
The retail business: price, service, selection: pick 2
I came up with the computer OS triangle of options: Stable, customisable, user-friendly.
The three possibilities separated rather nicely into Linux (stable/customisable), Windows (user-friendly/ customisable), and MacOS (Stable/user-friendly).
From the defense contractor/aerospace world:
Fast. Accurate. Inexpensive. Pick two.
Outdoor / Backcountry gear
Strong, Light, cheap
I find it useful when thinking about a purchase.
When living in/working on an old house-
straight, level, or plumb.
Usually, I get only one of the three.
We boiled it down to two. Do you want it now, or do you want it right?
Fast horses, fast guns, fast women. Take your pick. 
Medicine: good, fast, thorough
I used to tell my dad when he would nitpick:
You can have it done YOUR WAY, or you can have it done BY ME.
But you can’t have both.
I had a friend in college who had a square variation: homework, sleep, social life, extracurriculars – pick three.
I used to have a button with a variant:
On Time
On Budget
On Mars
Pick Two
From eating/cooking: cheap, convenient, or healthy. Any two.
An IT manager I know used to tell the executives that there were four variables they could manipulate on his software project: time, resources, scope, and quality… and that quality wasn’t negotiable.
Strong, light, inexpensive. I first came across that in a very old web post (which would have been a blog if there had been blogs) by Keith Bontrager about mountain bike handlebars. The ramifications of choosing incorrectly (let’s say light and inexpensive), well, there doesn’t seem to be a toothless smiley.