Hi Clark,
I was in a similar situation to you at the start of this academic year. I'm currently one of two club captains for Uriel (Exeter Uni Ultimate) and we too had a massive influx of fresh (whether it was because it was the cheapest AU society to join or because we dazzled them with the vids of Ultimate we had playing who knows...).
A massive number will naturally decline, people take up ultimate as something to try out and it's not vor everyone! We had 150 sign up at our freshers squash (with more joining throughout the early weeks) but now we have around 40 or so regulars and thats a more managable number (particularly for teaching a new sport to people when there are only 5 people on a team - speaking about Indoors clearly!).
But I digress...
a. The way we tried to attract people was to show how we are different from other sports at uni! The whole "lash-crazy, pick on the fresher's" attitude is something we stayed very clear from. The first few weeks were totally fresh based, introducing them to the game and making sure they had play time! For the first few weeks of training I don't think I actually played a point, instead I went around to groups of people who weren't playing and just talked to them (both about ultimate and just uni in general). I think that helped me come across as a friendly guy which in turn made the club seem more welcoming!
b. We held a social in the first week (our tradition is Heroes (experienced) vs Villains (fresh) where we tie - lightly! - an experienced player to a fresher). This furthers this image that our society isn't just based on the (competitive nature of) sport but the social aspects of it (which I think is massively important for the sport in whole!). During the first few weeks Freshers are all up for making as much friends as possible and just showing the club to be welcoming can do wonders for the amount of people who stay with the club!
c. If any of our committee were out on a night out and saw someone we recognised from training we would go say hi and have a little chat with them! Once again reinforcing this image of the club being welcoming to freshers! Make friends with people and they're more likely to stay... Their love for the game will come from Tournaments, which leads me to....
d. BEGINNERS TOURNAMENTS! You MUST get your fresh involved in a non-competitive tournament ASAP. We generally take 4 teams to a tournament in Plymouth and a high percentage of those who attended that this year have remained in the club! The best way to appreciate ultimate after all is in the tournament setting so you must find a local tournament at the start of the academic year!
Feel free to ask me if you want any more suggestions!

Also, what uni are you playing for?