-Lean out in the turns. You want to push the outside wheels into the ground, the karts are so light that weight distribution is a huge factor in grip. Watch beginner kart drivers, see which direction they lean, then watch someone that knows what they are doing and see the difference. Body language is a far greater factor on speed than actual weight. Of course, all else being equal, the fat guy is slower.
-Most beginners will tend to overdrive the karts going into corners way too fast or not braking enough. (the karts that most posters here have driven don't have front brakes, which are tricky, because you want to maximize braking early which is counter intuitive to street driving....though is also applicable to formula cars with downforce). So advice that Evadman or Fobot gave would be wrong.
-Most karts that the posters here drive simply don't have the power for point and shoot driving. No cheating here and counting on the machine to save you. Lot of guys try to get real deep into the corner, turn, and try to power out. Works on the street because people drive different cars. Foot down to pass, seems simple enough. But when everyone is driving more or less equal cars with equal power, that strategy doesn't work...and a lot of people get frustrated because they don't get any faster and blame it on weight or lack of power.
-Finally, it can be discouraging when you are driving at your limit and still being passed, and both people are driving equal machines...that there are faster people out there that you will never catch. Lot of guys take this as a personal attack on their ego. I remember going out against a few pro drivers (ie guys paid to race) and facing the reality that no matter how much I studied or practiced, I simply didn't have the talent to beat these guys. Most of them were kids...and its a very humbling experience.