In all seriousness, like has been said in this thread, there are probably 3 types of people who stare at you:
1) Those that are zoning out. Ignore them, they mean no harm.
2) Those that want to bang. If you are interested, initiate contact.
3) Those that want to fight you. A staring contest isn't worth getting in a fight over. Let them know you see them, but don't glare back. Keep a discrete eye on them and be prepared for a confrontation.