What is a committee looking for in a dissertation proposal?
JeffE is correct; talk to your chair and get feedback. That being said, you'll want to include the following in your talk:
- An overview of your research topic
- A discussion of relevant literature (to demonstrate that you've researched the topic)
- A discussion of your specific problem
- Another literature discussion, much more focused on your specific problem
- Any preliminary results of experiments you may have run, if any
- What you plan on doing for the next few years
- (Possibly most important) Any specific issues/problems that your advisors have raised about your research. You should specifically address these to show that you're paying attention to what they say and that you listen to their comments.
The best way to prepare is to talk with everyone in advance and find out what their objections/comments/issues/questions will be as much as possible, and then do your best to prepare for that and any related issues. If that sounds like a lot of work, it's because it is a lot of work.
Dan's comments are also spot on; you may have some very difficult committee members who will ask all sorts of questions, and you should make sure your advisor is on your side from the get-go.
Ultimately, your committee has total say about what constitutes a successful thesis proposal. As JeffE mentioned, you should ask them (particularly your chair). In my case, I scheduled short meetings (10-20 minutes) with each of my committee members. (Ideally, this should happen at least 3 before your proposal.) I asked each one "What are you looking for in my thesis [proposal] for you to judge it as successful?" I don't remember all of their answers. From three of the four members of my committee (including my chair) I had clear expectations, which I knew I could meet.
Sadly, from one member, I never really got a clear answer. Not surprisingly (in retrospect), part way through my presentation he complained because he thought that what I was presenting was off-topic. In the end, everything was okay. However, I think he was a bit unhappy and had to be persuaded to pass me. The single most important thing you can do to guarantee a successful proposal (or defense) is to make sure that your chair will strongly support you if you get resistance from another committee member. Besides that, you should schedule meetings (as I mentioned above) to ask what your committee is looking for. Start with your chair and get advice from him/her about how to approach your other members. If you don't get a good answer from one member, try scheduling another meeting to ask again (perhaps after meeting with your chair a second time to ask for help on how to approach the troublesome member).
Edited to account for EnergyNumber's comment. My own experience (mentioned above) was actually with my thesis defense, but I think the advice applies equally well to a thesis proposal, so I've edited my answer to address that (since it was the subject of the question).