Lights, Camera, Action (Landing lights on, Transponder on, Mixture/Prop/Throttle full forward).
Yes, due to heavy traffic. I leaned after the run up and then forgot. Not good!
If you've done all the runup checks and then sat waiting a long while due to heavy traffic, it wouldn't hurt to do a mini-runup again when you enter the runway. Just a quick engine check at 1700-2000 RPM for magnetos and carb heat before pushing in full power. This adds less than 10 seconds to your take-off, but may well catch a fouled plug or carb ice at a very critical moment.
The other thing is to keep a count. If I need to wait for a long time after the runup checks have passed, I might lean the mixture and I might also open the canopy for ventilation (Robin DR400 or R2160 with no other means of ventilation while on the ground). I mentally keep a count of things I need to undo before take-off. So when I finally line up, I know I'm two items short of being ready for departure.