This is seriously one of the most difficult parts of playing jungler.
So, somtimes you'll play against a good player jungling as a jungler. He manages to gank successfully, and generally beats the crap out of your teammates. So what do you do?
You CANNOT let him gank your lanes without punishing him. Take his creeps, counter gank, gank other lanes, and take objectives, but YOU CANNOT SIMPLY LET HIM DO WHATEVER HE WANTS.
Well, that's the basic idea behind it, at least.