Avoid building on level sites if possible. They are better for gardening and much more challenging to keep dry. Obviously there has to be somewhere for the water to go, so if you must build on the flat, you'll have to create a place for water to flow to. Make a ditch all the way around the house, draining into a big hole filled with round river stones and/or pumice. This will hold the water and let it soak slowly into the surrounding ground, away from your house or into your water-loving garden planted on top of the drainage area.
In Eastern Nigeria, people solved the problem of building on low, flat, wet land by piling dirt into raised platforms and making their cob homes on top of these, out of the wet. If you do this, remember to tamp the dirt often as you build up the platform.