If your goal is a sore bottom, I’d recommend spanking yourself for a longer period of time, rather than attempting to smack harder. Get into a comfortable position, set the timer on your phone for 5 minutes, and spank until the time is up. Then go stand in the corner arms folded behind your back, to let the sting develop and your arm recover. No rubbing, of course.
If you need more, get back in position, reset the timer and continue spanking for another 5 minutes. Repeat until your bottom is as sore as you want, it shouldn’t take too long.
An alternative to the timer is to spank yourself whilst watching a spanking video. Here you’ll give yourself a smack each time the on-screen protagonist is smacked. If you need more, rewatch the video or find another to continue the story. The self-spanking challenge series has more suggestions for introducing variety and interest into your disciplining.
As for ways to increase your humiliation of your punishment, I’ve compiled a handy list of activities here.
Hope that helps…