I like the line Neo Badness has on his sig, goes something like 'I spend my money on petrol and parts, the rest I waste'. Sums it up nicely.
It does require a bit of sense, you have to look at what money you have and what you need to spend to ensure a suitable standard of living. I spend a bit of my wage on parts for my car every month, as I do with my other main hobby, but I have to limit it and not forget what else I want to achieve. Meaning, I need enough left over for fun, like fuel for pleasure drives, weekends out/away, race entries (MTB), holidays etc. So budget!
Just remember, if you car is working, and you dont really have the money to spare, then you dont need the parts!