Ramming

Ramming means hitting the enemy with your own robot, rather than bullets. Ram damage gets twice the points and bonus as bullet damage, so it is a good idea, however it has to be done carefully. In order to ram you have to head almost directly towards the enemy, making you vulnerable to HeadOnTargeting, so it is usually best to wait until the enemy is disabled before ramming them, this way they can't fire (they also can't move, so it is easier to get to them).

The other point to remember is ramming damages you both (according the the FAQ both robots take 0.6 damage, which means you get 0.6*2=1.2 points), so if you are equal in energy or have less than your opponent, it is rarely a good idea. Again, this suggests waiting until they are disabled. However, if you find yourself near the enemy anyway, and have more energy than them, it might be worth ramming them, as you are already at the disadvantage of being so close, so may as well benifit from it.

Ramming is a dangerous strategy in Melee because, although your target may be disabled, there are still other bots that can deal serious damage. Of course, if you are the last two robots left, it is really a OneOnOne battle, so ramming works.