put something in your hand (something that won't cut you, of course) ... like keys (though be careful with them) ... or something ... and your hit will be much harder ...
also, if the person doesn't look really strong, hit him in the stomach as hard as you can ... and then hit with your elbow in his back (disclaimer: you can damage his spine like this ... so , be very careful if you know this person) ...
when the stomach hit doesn't work, hit him in the nose, or somewhre in the face ... and start hitting him everywhere while he tries to feel his nose ...
also, the hit in the knee is a good one ...
as was mentioned before, the element of the suprise is the best tactic...
...
if you don't really want to hurt him, take his arm , and like twist it behind his back, then throw him on the ground (while standing behind him ... while he doesn't have time to kick you with his leg) ...and put your knee on to his spine ... and if he tries to do something - press your knee harder and twist his arm harder ... that will teach him a lesson...
if he's much bigger than you and stronger, then don't think that you will look like some girl , just hit him in the balls as hard as you can .