Why, they named the game Tetris, not Triple, nor Double, nor heaven forbid, Single. So don't waste your time with little pee-wee scores and just go gung-ho for the big four-score.
The basic strategy is this: leave one gap on either side of the field, build blocks up in the meantime, and wait for a straight tetrad to come along to eliminate said rows.
For more incentive to score Tetrises, take a look at the Points Table.