I have made that mistake a few times too often.
Now I have started to use the Mode switch on my DX18 to set 3 modes.

mode 1 is Ground, and the gear will not retract
Mode 2 is TOGAL (take off, go-around, land) and everything works
Mode 3 is cruise, and the flap will not come down.

I start up and taxi out in Mode 1, stop before entering runway, perform pre-takoff check (including gear selected down)
switch to Mode 2, enter runway and takeoff
after climb and cleanup I switch to Mode 3 for fast passes and aerobatics.
When it is time to land, back to mode 2
after clearing runway, back to mode 1

the throttle interlock could be a good idea too. I have not thought it all the way through yet.