If you want to mulch you should never cut more than 1/3 the height of your grass off. So if your mowing height is 3", you can't let the lawn get higher then 4.5" before mowing. Also it really helps if the lawn is dry and your blade is sharp (not necessarily razor). Grass is 85-90% water, so the blades left behind will shrink to almost nothing once they dry out. The last thing I would suggest is to slow down. Give the mower time to cut the blades multiple times. If you do these things most mowers with a mulching deck will do fine.