Do Your Research
Research Prices
Spend some time getting a idea of how much your chosen bike is worth.
Ebay Using Ebay's "view completed auctions" can give you an idea of how much similar bikes have sold for.
Parkers Guide is a great resource for getting an idea of prices and will also give you useful information on the bike
Get an Insurance Quote
Make sure you get an insurance quote! before you get your heart set on a bike or even worse, buy the bike of your dreams only to find you can't afford to insure it!
Bike Specific Problems
Some bikes have 'common' faults. Early R6s are known for eating the gear dog for second gear causing the bike to jump out of second gear. Ask dealerships, bike-savvy friends, and use the internet to research your chosen bike.