I used Cycle Trader to bargain my way down $500 on a new Speedmaster back in 2003, the first year for that model. Make sure they aren't charging you large amounts for shipping and dealer prep.
Always, always, always...aftermarket bags and windshield. Stock bags are too small, and a large number of people experience buffeting with the stock windshields. The tachometer I would also skip Triumph's model. Drag Specialties is a popular one you can mount where you can see it, and it only costs about $60.
Progressive shocks are the most popular brand for an upgrade. Part numbers are in the Tech Vault, and testimonials are everywhere by doing a forum search. There have been lots of inroads on accessories for these bikes that end up better looking, better performing, and longer lasting(especially bags) than what Triumph offers.
Sissybar and luggage rack you should be able to get at 15-20% off when you buy the bike. Might try a WTB post in the classifieds first though to see if somebody wants to get rid of theirs. That is the one accessory item I went with Triumph.
Best tip is read in here before you buy anything extra for the bike. Could save you a nice pile of cash in the long run.
