Before the flop i bet in multiples of the big blind, some of the factors to take into account are my hole cards ( is this a calling or raising hand ), my position, how many other callers are in the hand, has the hand been raised and by whom. The amount i bet varies from hand to hand even if i have the same cards because other factors will be different and i want to disguise my hands.
After the flop i bet in terms of the pot and the number of factors involved has probably increased, this is so dependant on the individual situation that advice is almost meaningless. The best i can think of is that your reads on the other players will give you the biggest chance of making the "correct" sized bet.