In addition to the rule of "buy it when you see it", here are a couple more tips...
Bring cash! It always helps when negotiating. I've found that most members will trust another member to write a check, but cash always works. Also, bring an assortment of bills - if you negotiate a price of $100 down to $85, don't hand over a $100 bill - it can make the seller feel that he should have held out for more. You never know when you might deal with that person again.
Before you buy from a dealer, ask if there is a discount for cash. Many dealers will offer a discount or "pay the tax" if you're paying with cash. However, I've noticed that an increasing number of dealers don't discount for cash, possibly because credit card transactions have become simpler or it syncs up with their bookkeeping software - if there's no discount, whip out the credit card and get your points, and conserve your cash for other purchases.
