As everyone above! Smooth driving on the racing line.
Keep it moving. Leave the spinning to the others.
If yr coming into a corner with loads of other karts, try to get the inside line before the corner and push the others out wide.
If you are racing with a bunch of people who haven't raced before - or very much then you can usually bully your way up the inside.
Experienced karters tend to be a bit smarter than that.
If yr stuck behind someone, follow them for a lap or two and work out where they're likely to go so you can decide where you're gonna nail them!

Finally - blag a go in something like a rotax 2 stroke. You'll shit yr pants!!