Unless you run the motor aground in mud, or use it in some kind of strangely polluted water, you should not need to flush it. Again, you are in a freshwater lake, so there is no reason to flush it for corrosion protection or the like, if you were operating in saltwater.
Having said that, you can buy a motor flusher device at Wal Mart or any boat shop for a few bucks. They are commonly referred to as "earmuffs", and attach to a garden hose. You then put them over the water intakes, turn on the water, and start the boat.
