Always look ahead for your next move. Remember: if you run out of valid moves, the game is over. So check carefully to see whether your move will open additional match options (good), reduce your available matches (bad), or leave you without any further matches at all (d'oh!).
Also remember that matching a set of gems on the bottom of the screen will definitely affect all the gems above it, so if you can see two valid matches, consider the top one first.
Combos and cascades score extra points! The more combos or cascade matches you can make with a single move, the more the bonus points rack up. Look for moves which will either make multiple matches at once, or cause the gems above to fall down into more ready-made matches.
In some ways, Timed mode is actually simpler than Simple mode. If you run out of valid matches when playing Timed, it doesn’t end the game; instead, the gems will be replaced with a fresh set and play will continue.
When playing Timed mode, however, you may want to avoid filling up the bonus bar. This is counterintuitive, yes, but there are two practical reasons to consider it. Once the bar is filled up, A) many gems are removed from the screen, causing an unpredictable cascade effect that can easily break your concentration, and B) the bar is reset to a lower level, which reduces the amount of time you have to make your next match. When the bar gets close to the top, you might want to take a few seconds to look around the board and plan your next moves.