I think about all the money I've spent on my car a lot. But I know that it is something that I'll never get out of. And I have spent my money on a lot worse things than car parts (drugs/partying) in the past. Lately, I've been considering selling my car and getting a Grand Cherokee or something similar, but I know as soon as I do that I'll be buying wheels and tires for it, lift kits etc etc.
Kinda like NINaudio said, the way I look at it is a car isn't really mine until I've given it some style. Anyone can roll around in a boring *** stock vehicle. And if you think about it a good majoity of cars on the road nowadays are modded in some way. Even if its just spinning hubcaps.
My secret to not over spending on my car is just being patient, and waiting till I find that deal that is too good to pass up. Then I buy. That is why I dont have exhaust yet, or coilovers, because I know that eventually a great deal will come. I have nothing but name brand parts on my car, and I have spent very little money on those parts. Granted my car is not anywhere as heavily modded as some of the cars on here, but I do get plenty of complements on my car, and that is good enough for me.