Basically, try to drive perpendicular to the GM. When it starts to turn, turn with it so you are still driving perpendicular. You will need to continue this (basically driving in a circle) until the GM hits the ground next to you. Watch for walls; if you get blocked from continuing and the missile still has a clear shot, you're dead. This method is very fun when a player shoots five or ten GM's in a row at you and every one misses by a fraction of an inch. It takes some practice, though.
All this is assuming you're on open ground. If you can dive behind a wall, that's better because it takes less time, but watch the height of the person firing, as it may be able to pass over the wall. (Also, just because there's a wall between you and the player doesn't mean you're safe: you can sometimes fire to the side of the wall, then lock to make them turn the corner.) Never jump, because the missile will easily aim and hit you. Once you've jumped (except in the rare case that your jump got you safely behind a wall) you have no chance left unless the lock is lost. If you manage to kill a player (or someone else does) all locks are lost and you can safely jump if that's the best way to dodge it.
As Bambino said, don't even bother using the first method on servers like MW 2.3, where shot and GM speed is higher than usual. Without a flag to help you or dodging behind something (or in a pyramid), you can't dodge it. However, if you are very far away, it may time out before it reaches you, so don't give up.
I've also heard about using a twitch (see the link Bambino posted), but I haven't tried that more than a couple of times.
Whew, long post. Hopefully people can link to this in the future if others don't feel like searching
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