The best way to do it is use infinite boost in a time trial race in Wellspring. After that buy the miniguns and get the bandit quest from Sandy. Go out and kill a bandit and then check your inventory. There should be a skull in your inventory. Use the Mega Items cheat that is in the trainer and then you should have 99 skulls.
Trade them into Sandy at the bar and you should get 99 racing certificates. That is how I got all the upgrades for my car.
[Edited by fullmetal5550, 10/10/2011 12:16:38 AM]
Of course I read your post. The issue is, that I don't want 99 of everything. I prefer to use cheats to get past certain aspects I don't enjoy, rather than ruin the entire game by using them excessively. Otherwise It would be simpler to use the command console.
This wouldn't be such an issue if I could drop my inventory like an RPG, but there isn't much option here.