I'll try to answer your questions ...
To keep a deck usable longer make sure to wash your hands, and dry them thoroughly, before using them. Since you short on money, I wouldn't suggest buying a porper clip, but it does help decks last longer, though it's a luxary, not a necessity, for me.
I keep a practice deck and a performance deck; I have had my practice deck for 3 months now, and my performance deck for 1 month. It's all based on feel -- if you feel that the deck is beginning to stick and clump together, you may want to switch your performance deck, or keep it if it's your practice deck.
I perform with different decks all the time, and since your a poor college kid, like I am, I suggest going to Costco, BJ's, or Sam's Club and getting a 12 pack of Bicycle Decks for $15. I bought it 6 months ago, and have only used 4 of the 12 decks.
Hope this answers all of your questions. Let me know if it doesn't or if you have more.
Cheers,
JTM