I never figured out why group projects always seemed to go well with me (post-high school, anyway), until three terms ago. I overheard one girl asking one of my groupmates how we had put together such a good presentation, and my groupmate just said, "You've gotta have one member in the group everyone respects, and that person has to expect the best out of everyone. Next time, just approach someone you respect."
It took me a good fifteen minutes to remember that she'd approached me. I don't know why she respected me (or whether anyone else in the group did), but I think her advice is pretty solid:
Pair yourself up with people you respect.
If you're a good judge of character, the people you respect won't let you down. I've always tried to pick groups made up of people I liked/respected, and I have never had a bad experience with group projects.
As for your situation, I think telling the professor ahead of time was a great step. If he grades you down for the lack of work your peers put in on the project, I would recommend you argue for a better grade in the following manner:
1. Approach during office hours with your prep in hand (papers, etc.)
2. Ask (don't demand!) for an explanation of his grading/a project rubric that tells you how individual members of the team are assessed
3. As nonconfrontationally as possible, ask for his opinion on your part of the presentation: was it on the same level as your groupmates? Did you seem more or less prepared than the others? Had your groupmates done their part at the same level, would your individual grade have been higher? (Note: this is where your prep comes in. If he has little to say/negative comments on your preparedness, you can show him your work and ask what could have been done better. A reasonable teacher will see the discrepency and, nine times out of ten, award points as is appropriate)
4. Regardless of the outcome, do not outright insult your groupmates or whine about it. Simply ask that, should there be any further group projects, you be allowed to work with a different group.
This is essentially an alteration of my usual approach for seeking grade alterations/explanations on projects. I've never had to use it for a group project, but I will tell you that this works for me about 95% of the time. The key is not to be pushy or confrontational. Come anywhere near your teacher's ego and you can kiss that better grade goodbye.