I think forgetting to raise the flap has got the be one of the most common things. I was recently out with an ex-Navy, F-18, Mustang and warbird flying CFI with tens of thousands of hour and he forgot it as well, so it's not just us low timers..
I'm blessed in one way, and that is that my gear extension speed is 156kts, and that gear extension is the first thing one does when one wants to start to slow her down with the geared props. It's so early in the chain that it's not easily forgotten.