Psneeld makes a good point about impellers taking a set. We have two pieces of equipment on our boat that use flexible impeller pumps but don't get a lot of use. One is our Onan Jurrasic Model MDJE generator and the other is a huge (for our kind of boat) Westinghouse AC salt water washdown pump.
On the advice of a friend who's a retired marine engineer we run the AC pump every time we're at the boat for a few seconds. The killer of flexible impellers is heat, not running them dry. Of course if you run them dry it will build up heat extremely fast and damage will occur. But running the pump for two seconds or so hurts nothing. However it not only puts the impeller in a different position but it spins the pump shaft in its seals and so keeps the shaft and seal surfaces smooth and in good condition.
On the advice of our local Onan dealer we bump the starter on the Onan when we're at the boat even if we're not taking it out or using the generator. This keeps the starter solenoid limber, turns the starter motor a wee bit, and moves the raw water pump impeller enough to put it in a different position. Just a bump. Not a turn-the-engine-over thing.