Happens on every flight. Forget to turn booster pumps off, forget to set DG, forget to turn off landing lights etc, etc. Last time I flew for 15mins before I remembered to raise the flap... and I've flown 50nm once without raising them and it was only when I was going to lower first stage of flap in the circuit I realised they were already down! I did remember thinking the airspeed in cruise was low...
I use checklists for start, runup and before takeoff, but not in the air. I suppose I should, but I don't think it's all that safe in a high traffic environment. That said, I rarely forget the before landing stuff (as it's so simple on mine), it's always the cleanup stuff after takeoff I botch. I should use my after start checklist more, really. There's no excuse.