HOW?
Simple, look for the lines.
Anytime you end a combo, just look around where you stopped and you'll see at least three or four other things you could have done easily. After a while once you get the hang of spotting things and picking the right tricks, your combos will be huge, just push yourself a little more each time.
For example go to London, there is a line that I found that can get you 6,000,000+ points if you get it right.
I think the College is the best place for terrific lines, with the Carnival coming in 2nd. Of course, nothing can match Rio in THPS 3. THe best combo/run I ever had was 235 tricks, netting me somewhere in the neighborhood of 16,000,000 points.
On my buddy Keith's THPS 3, we were playing one night, when we realized that the meter was always on special. We had never entered any codes, but the meter was on special all the time. Doing runs with the ability to bust off your special tricks at any time is a good thing.