Something I've been doing for the last few years that seems to help defray the cost of replacement parts and ugprades is to "pay" myself for going riding.
Everytime I go for a ride, I'll stuff a few bucks into the otherwise useless pocket on my underseat toolbag. Over the course of a month or two, depending on how much I've gotten out, I can save enough for some new tires, replacement brake pads or whatever my bike (or bikes) need at that time.
Not rocket science but it has been a pretty useful to me. Plus its also nice to have some cash on hand if you're out and need to stop for a snack or drink.