As of yesterday, I officially have a full committee of people who have agreed and are interested in my project. Huzzah!
My program is a bit odd in the requirements to advance to candidacy, in that you don't have to do everything at once. At this point, I've passed my qualifying exams/coursework, and the next major hurdle is the preliminary proposal of thesis work. We have to put together at least part of our thesis committee at this point, though it's highly recommended you try and find a complete committee. Since this can be a daunting process, I felt I should share my steps to finding my committee members.
Step One: Look up your committee requirements
How many people do you need? Are you required to have certain balance of members inside and outside of your department?
Step Two: Identify the types of expertise you need
If you're doing computation on a particular system, such as in my case, is there someone doing experiments on a similar system? Are the other people doing similar experiments on different systems, if you're an experimentalist? If you're doing something that is new to your lab, is there someone else at your university using similar techniques? Faculty websites are a fantastic way to look up this sort of information. Most importantly, have a list of more people than you actually need for your committee. Professors are busy people. They may not have time to be on you committee, so be sure to have backups (but never tell them they are your backup!)
Step Three: Set up a meeting
This is, for me, the hardest part. What do you say? I've been approaching this with the idea of "Brief but thorough". My emails have looked something like this:
I am a student working with Pf. Blarg in the process of putting together a thesis commitee. My proposed topic is BasketWeaving, which relates to your work in FiberPreparation. Would it be possible to meet at sometime in the near future? I am unavailable ___, but otherwise flexible.
Step Four: The meeting itself
Sell your project. If you don't sound excited about what you're doing, they have no real motivation to agree to invest time and effort in being on your committee. It's also important to be clear on exactly why you think they would be a helpful addition to your committee (see Janus for the professorial side of this issue)
Once you have your committee comes the fun part: scheduling!