I do it by race results. If I have a good race (no DNFs or injuries) I can buy everything I need for the next race (tubes, chain, tires, race entry, etc), and if I have a great race (category win, huge
PR) then I get something nice. Last race I podium'd I got to replace my 6 year old mountain bike helmet, and I'm hoping for a skinsuit this summer, it keeps me motivated, cause I really want to stop TTing in a long course trisuit.