Sorry you didn't get a response sooner. What many people do is have a drainage layer on the bottom like these terraria:



I run a siphon hose down to the drainage layer when there is too much water in there, and just siphon it out. Often it helps to have a small section of pipe incorporated into the drainage layer and through the soil so that you can easily siphon the bottom whenever you like, just by sticking a hose in that pipe. It's harder to do this once the terrarium is built but it sounds like it may be advantageous for you in the long run to do something like this. You may need to put some holes in the bottom of the pipe section to allow free water flow into it.

An alternative I've heard of is to run a shoe lace into the drainage layer and leave it hanging over the side of the tank with the end lower than the water in the tank - through capillary action it will slowly soak up the water, eventually dripping it on the outside lower end - effectively a slow siphon. Just add a bucket.

The first method is going to be a lot quicker though.