You are MUCH more likely to get a cheap deal if you go mid-week. Package deals offered by airlines and places like expedia are often pretty good. I've heard priceline is good for the higher end strip hotels, though I've never used it.
There are certain times of the year - during big events in Vegas - when hotel rooms are at a premium. These include NASCAR weekend, Super Bowl weekend, anytime there's a title fight, etc. During these times you will NOT find $29 rooms! Even the sleazier downtown joints will be at least $75. Of the strip hotels, Stratosphere is where you'll find a reasonable room when everyone else is either booked or charging an arm and a leg.

