Once you have all the water out that you can get by pressing/kneading with your hands, you can put the butter in between two sheets of saran wrap and use a rolling pin to press out even more water. The more you get out, the longer your butter will stay fresh.
Another thing I've learned is that puncturing a Vitamin E capsule and adding the contents to butter helps preserve it. Do this after getting the water out. Salting it helps, too.