water heater check up/maintenance

[ponch37300]

I believe that my water heater is working alright but sometimes when we take a couple of showers in a row we will run out of hot water. Think it's just a small heater but wondering if there is anything I should check or do as far as maintenance goes? How often do you replace the elements and thermostates? Is there a way to test these things or anything I should look at or do? Thanks, Scott