Like others have said just don't buy games as soon as they come out. It's really crazy how much a game can fall in price in such a short time.
Wii is doing fairly well but you can pick up most games that came out last year / early this year for under $35 including gems like Excite Truck, Trauma Center etc.. (i'm going by ebay.com listings here)
If that isn't cheap enough you could do the hardcore solution and not buy any current gen games or consoles at all! Build up your collection of past gen titles for next to nothing - I guarantee there will be loads of fantastic games you have yet to play!
edit - Oh and speaking of retro stuff... car boot sales / garage sales! Maybe not so great for current games (at least in my experiences) but older stuff and retro stuff is a potential gold mine. If retro isn't your thing then buy them anyway and sell them for a profit on ebay or something. Use profits towards a new current gen title!
Of course if you want to just play more games on the cheap, you could always temporarily swap games with friends (unless your friends are the kind who love to scratch up discs).
My parents used to do that with the NES when I was a kid and swap our games about with other kids parents. We'd end up swapping our whole collection about - it was fun waiting for my parents to come home seeing what games they had brought home. One time they bought home a huge cartridge rack with about 20-30 new games to play, that kept us busy for a while!
Nowadays there are quite a few US sites offering swap services e.g.
http://goozex.com, which might be worth a look.
In the Genesis days I would get to play lots of games by trading in my old ones and getting something different. Back then you could trade in a game, get a decent trade in value for it and get another game. If the game sucked you could trade it back in towards another title and get virtually what you paid for it back. Nowadays trade in values are teh sucky to the extreme, but instead of going to a game store you could buy a game on ebay and once you tire of it or whatever chuck it back up on there and get your money back minus shipping. This really isn't too different from rentals I guess....
