Shifting up:
Wait untill you get to the limiter(to find where the limiter is just rev the car up)and shift right before you hit it. This helps on those cars with really high powerbands(like all of the Hondas lol).
OR
Wait untill you get to the red line. This usually works fine for most cars.
OR
Find the end of the powerband. This can be done by looking at the max power(both horsepower and torque)and seeing at what RPM are they at. Also with some of the turbo cars(this is especially for the SLK), you will notice at a certain RPM, the boost needle will drop off, and you won't accelerate as fast as you did when you were at full boost. So in that case, you shift when it starts dropping off. With V8, V10(Vipers), and V12 cars, most of them loose power before the redline, so again you will just need to look at where the power is, and shift accordingly.
Shifting down:
Make sure that you have enough RPMs left to downshift. Other wise, you end up over revving and slowing down when you were trying to go faster. For braking, it really doesn't matter...I usually kick my car into reverse(just tap reverse while slowing, be off the gas)and that puts it into neutral, and then when you need to accelerate, just hit the gas and you will be in the right gear. You can also use your engine to slow down by shifting into a lower gear, but over revving does not help, you need to be in gear.
Hope that helps too