Various Strategies
Focus on a single color to take out first in order to allow the other color create longer lines. Watch the pattern of the level and check which color would benefit you most if you removed some balls in the long run.
Move quickly, as waiting too long for longer lines will result in balls hitting the lasers. This is especially true when new balls are falling; catch them before they bounce away.
Only use the magic balls that allow you to continue a line with a ball of a different color towards the end of the level. This is also the time when there is a better chance for more balls to be on the board, creating longer lines.
When feasible, create as many long lines as possible. The longer the line, the more points you will score.