How do you reverse an aikido throw?

In Aikido, or any other grappling arts, there is always a way out or an escape. Your job is to make that chance as small as possible as the one doing the throw. Since there is an estimated 3000 combinations of Aikido Waza, you would have to be more specific as to what your trying to get out of. If you're already being thrown, you're going to get thrown. Usually it's your momentum that is what makes the throw possible. Easiest way to get out of a throw is to not attack.

----------------------------------------------------------------------------------------------

The answer Aikidoka gives is absolutely correct.

However, there is a specific training in Aikido to reserve an action (throws, arm-locks, technics ...). Those are counter-technics and experience is needed to practice those.

It is called Kaeshiwaza. There are almost an infinite of possible combinations.