I agree with Jim on the convex mirror. I bought the round, 3 3/4" version ($2) and stuck it on my Raydyot side mirror. Gives me a great view of everything that would normally be in my blind spot. Between that and my "normal" rectangular rearview mirror, I can watch all those mere mortals disappearing in my wake.
The second best accessory--though quite a bit more expensive--was the lift I installed in my shop last year. It has been invaluable in many projects, large and small. It sure is nice to have something at eye level as you stand while working on it, rather than laying on your back on a creeper--