"When waiting for a Tetris opportunity, there is inevitably a screen full of blocks just waiting for a long piece to slip into the gap on the side. What's helpful is if you build from the side opposite of the gap, since it will ensure that there will be room to maneuver and rotate the long piece into the gap. It will look like a jagged ski slope decending towards a cliff (left). This tactic can save you from overloading the screen because you can use the T, L, Gamma, Z, and S to eliminate a couple lines while waiting for that long piece, but at the same time not closing off the gap, since there's nothing worse than closing off a gap right before the long piece comes." (submitted by Khoa Vuong)
There's actually a mathematical formula to determine the optimal Tetris curve, but, er, I don't want to waste your time with such technical mumbo-jumbo.