Here is some pics of the design that works well for me.
http://picasaweb.google.com/jpwiggo/BatHouse?feat=directlink
I've seen some designs on the net and they look easy to build, but for me my wife always seems to get me one for Father's day or my birthday, so have not yet had to make one. The thin houses with hardware cloth or plastic window screen attached to the back wall work the best for me. You can see the design I'm using in the pics. I put my houses 10-20 feet high, on the WEST side of the field, facing the EAST.
I live 150yards from a Bog (swamp) and bitting bugs are the worst here, we have tried everything - and the best solution is bats. The bats come out at dusk, and take care of business.
I'm in NE Ohio in my area we get eastern pipistrelle. After a bat house is up - sometimes they move in that year, but generaly it takes a year or two before bats move in. Often it starts with a single bat, and then the following year there is a full house. I don't know if it helps but I 'seed' a new house with some guano from the other houses. I figure a little smell helps the solitary bats find a new home. If anybody wants a teaspoon or two of guano - PM me your address - I'll drop you a bit in a ziplock bag.