I live about 15-20 miles away from the stations and there are a lot of trees around my house.  I am thinking that this is greatly affecting the signal strength, because right now its hit or miss whether the stations will come in, and if they do come in, they most likely drop within the first few minutes stating “signal too weak”.  Any suggestions on a good, strong HD antennae?

So you have line of sight, except for the trees?  Do you need VHF or UHF? Indoor or outdoor antenna?
Here is a TV Signal Locater and an Outdoor Antenna Comparison for your benefit and others reading this post.