There have been studies done (no, I'm not gonna go and cite them or even look them up again) that show that professors perceive that women talk a far greater percentage of the time than they actually do (i.e., if a female student and a male student both talk 50% of the time measured objectively offline on a tape recording of the section, the professor perceives the female student "dominated" the conversation and actually spoke about 80-90% of the time... if memory serves). All of which is to say, you may find your professor's perceptions to be very different than reality or your own perception. That said, you have to live in the world that exists until you have time to change it -- so here are a few thoughts:
1. Think carefully over what you want to say, and say it in as few words as possible using simple, declarative sentances. If it's not a question, don't ask it as one; drop your voice in pitch at the end of your declarative sentances. And I have to agree -- no passive sentances.
2. Make sure when doing point one that you speak loudly enough that you don't have to repeat yourself -- repeating yourself draws attention to how much time you are using, and also usually means you wind up talking over someone else. It also sucks when you say something, but not loud enough, and one of the male students then repeats the same thing and gets 'credit' for the idea.
3. Don't raise your voice (ever). If you raise your voice, it usually goes up in pitch, which makes you sound silly. It will also give some ammo to people who are trying to say you are being unprofessional (although men do it all the time). If you are being spoken over, focus on dropping your voice a bit (because it will probably go up in pitch when you get angry) and repeat yourself while staring pointedly at the person who is interrupting you. Alternatively, say, smiling at the person who interuppted you, "Jim, I must have sounded like I was finished. Let me finish this point, and then you can go ahead."
4. Sit at the front of the room, close to the leader of the section. This bridges the psychological distance. I think it works mainly by making the professor feel like you are having a conversation with him personally, rather than speaking publicly, thereby fiting more into the 1950s view of how women should communicate their ideas.
5. Have a close friend watch the interactions you're having, and comment on them. You need a very close friend to do this. Alternatively, use video or audio recordings to find out if there is anything that is particulary grating about the way you interact.
6. There are different expectations of male and female doctors, but you absolutely must get the best care for your patients, and being passive isn't what does that. You need to master a wide range of techniques, which, depending on the field you go into, may involve needing to take control of a room during a code, telling a subordinate "look this is an emergency, and you do exactly what I'm telling you to do." Cause guess what? Patients expect their doctors, male or female, to take care of them when they are sick.
The important thing is to make sure your voice is heard, and that the way in which you communicate your ideas doesn't alienate the people you are speaking to unless absolutely necessary. In the classroom, the stakes are low, but you need to practice for when you will be advocating for your patients (which will occur sooner than you think). Being a woman throws a bit of a wrench in the works, as you may not have the best female role models, but you should seek them out and try out some of the ways they do things.
Best of luck,
Anka