I have an earth box that was developed by a fellow Whisnant (he spells it wrong) in Florida. They work ok, but require frequent filling of the water reservoir for a large plant like a tomato. You can also get ideas from going to the 'Container' forum and doing a search for self-watering containers. There are plans for building your own from large plastic tubs available from Walmart, etc.

Here is a link that might be useful: Earth Box