What I would say regarding bumping, Is that if you dont have experience bump drafting then dont even try it. You can almost guarantee it will end in tears!
This is where practicing on the server is crucial. Bump drafting AI ALWAYS ends in tears because they seem to love to brake check you. However, practicing on a server, it might be a good idea to try gentle bump drafting...and like Lugnut said ONLY IN THE STRAIGHTS! Never run up fast on a car and slam his bumper. This will mostly end up bad for one or both of you. If you feel yourself in a good draft and fast approaching a car in front, drag the brake a little to gently connect to his bumper. Be aware of the front car and if he starts to get squirrely or a little sideways, get off his bumper. Also be aware of the corners. Near the end of a straight if you're hooked to a bumper, drag your brake a little to disconnect. Don't lift off the gas unless something's going majorly wrong...just press the brake pedal a little. Letting off the throttle will lose a lot of momentum you had.
During the recent Skoda racing we did, I had an awesome start and I forget who I was behind but I bumped drafted him all the way to T1 and we gained a couple positions.