Most of the time, we empty our boats on shore.  Most boats these days (Jackson Boats are an exception) come with convenient drain plugs.  If you haven't done so, I highly recommend tying a thin string from your plug to the end grab loop so you don't lose it.  I also highly recommend purchasing a spare plug and even pack a wine cork in case someone else loses a plug.  Carefully unscrew the drain plug.  Now roll your boat over on its side, this should remove 3/4's of the water rapidly via the cockpit.  You can now lift the other end of the boat to allow the water to drain via the drain plug.  Since water weighs 8 lbs/gal and the typical boat weighs 40 lbs, you are lifting roughly 40 + 8*15 = 160 lbs - better get someone to help you.  When you are finished, don't forget to screw the drain plug back in and perhaps mop up the remaining water with your sponge.

Another method that works is the T Rescue.  This is great on larger bodies of water and can save a great deal of time.  Basically, you have a friend drag your boat upside down over theirs to empty it out.  Here is a nice video from NRS on this  technique: T Rescue.