Hi pstine,
The soaring program is amazing and I highly applaud your desire to participate. To answer your question, there are a few things that are often considered when selecting upgraders:
1. As others have mentioned, GPA/MPA/PEA/OPA are important and have minimum cutoffs. OPA also determines to some degree who gets their first choice of summer program, so a reasonably high OPA is important to get into 251 as well as 461 (upgrade). This means you should strive your best to be a good 4 degree (do well on knowledge tests, follow the rules, etc.), earn good grades, and perform well on fitness tests.
2. If you get into 251, work hard. Do your best to fly well (it is considered), but even more importantly, be a good team player. Be outside pushing planes and helping others launch when you're not flying. IP's will take notice of who works hard when they make their decisions about 461.
3. Be humble and show a good attitude. One of the important things considered is whether or not a potential upgrader would be a good colleague and easy to work with (this factors into the interview as well).
4. Prior flight experience is another factor which may also be taken into consideration. However, prior experience is not necessary to become an IP and there are many soaring IP's who had none before coming to USAFA.
A couple more things--be careful not to get ahead of yourself too much. I am not saying you shouldn't have aspirations coming in (I did), but 4* year is a long road and there will be time later on to think about soaring. I would recommend spending most of your time right now preparing for I-day and BCT.
Additionally, all 4*s take an introductory glider program called AM-250 at some point during the school year. You will get four flights in a glider, including one aerobatic flight. This is a great time to ask the IP's themselves about airmanship, summer soaring, upgrade, etc., and also see for yourself whether or not you enjoy flying. So in other words, you will have a chance to try flying a glider regardless of whether or not you become an IP.
Good luck to you.