I have found the following routine pretty good each time I work out my Chest at the gym:
3 sets of 8 reps on the bench press
3 sets of 8 reps on the incline bench press
3 sets of 8 reps on decline bench press (get the decline by raising the front end of the bench with a block or one of those steppers)
3 sets of 8 reps doing dips (none assisted this is difficult at first but its something you get used too very quickly make sure form is good, get the chest pointing in and doing all the work)
3 sets of 8 reps doing dumbell flys
3 sets of 8 reps doing incline dumbell flys
By this stage your really going to feel the chest tightening up, so I then finish up with some cable pulls where you bring the two cables across your body using the chest.
The final exercise is more of a burnout exercise just get on the seated chest machine at the gym, set the weight to a level where you will be able to do 12 just about then imeadiately after each set lower the weight but keep trying to push out 10 - 12 without taking any breaks inbetween sets, after about 4 - 5 sets you will reach a weight that offers little to no resistance this is when you know your done and can pat yourself on the back for a good chest work out then go off and do some arms and leave the chest for another week
8 reps per set is good enough if you use a weight that your really pushing out the last 1 or 2 reps. Its also good to have a spotter so that they can give you a gentle touch on the last 1 or 2 reps of the last set as this is still better than doing 6 on your own because you still have to control the weight as its lowered then push it back up again, after a few months you will notice more strength and better definition as you are hitting the chest from all angles and giving it a really good all round workout.
The guy that I train with has seen massive gains in his chest, I myself have not been so lucky but it does show that 2 people doing the same routine one has made significantly more gains than the other so its a lot to do with genetics.