What size pot?

Further, you should fill pots with potting soil, not soil from the garden. Garden soil won't drain well in a pot.

2 things come to mind.

Potting Soil is better than garden soil, but potting mix is even better. I would reccomend AL's potting mix though, as being the best, especially in your situation where you will probably be having the eggplant in the pot much longer ( than our 4 ~ 5 month growing season ). Check out the CONTAINER GARDENING FORUMS for AL's mix.

The second thing that comes to mind is mulch, even in pots I use about 1 1/2 ~ 2" of mulch. If you have a windy yard it will dry up the soil at the top of the pot very quickly if it is bare.

HTH