Your basic goal is to ensure there's enough room in the playing field so that the thing doesn't explode when the next sets of tiles are shoved in. (In other words, in each column there needs to be at least two blank spaces, so the tiles can be shoved inwards without impacting on each other). As the levels go up, the amount of time between new tiles is shortened, you'll need to deal with more rows, and more colors are added at levels 10 and 20.