Always low reps, no more than 5 for lower weight and no more than 2 for higher weight. Also heavy negatives will boost your bench by insane amounts... Don't worry too much about keeping your body flat on the bench once you start doing heavy negatives, its all about feeling the weight and building the strength. Then when you go back down to your originak weight it will feel like a feather. Ive won heaps of gold medals and broken records and this style of training has never failed me.