To get a bit more money from hold-ups in the various shops located around the city, make sure to hit up the cash registers on your way out. And if you do it MY way, you'l get away with no police attention.
Get the money from the safe and kill the clerk. Make sure that you kill them with melee weapons and not by shooting them or there's a good chance you'll set the alarm off. If there's anyone else in the store (I've only done this with 1 patron, not 2), do them the same way. Beat the snot out of them to make sure they can't set off the alarm and then take one of your guns and shoot the registers. At least ONE of them will always drop some bills and change.
If you're going to do this, make sure you don't have a homie with you that may pull out their gun and start shooting. Once they fire their weapon, the police will be notified.