Engine size matters. Too small and it won't go up hills. Too big and it'll drink like a 747. Get the right body. Hatchbacks sell better than saloons.
Estates attract a premium.
Always negotiate with the dealer. If he won't give you money off, try to get a longer warranty, free servicing or options like air conditioning and metallic paint. It's a buyer's market, so make sure you compare lots of prices. Your part-exchange can be worth big money to the dealer. Prepare it well and don't give it away.