I've been using a wall too. It took me some practice before I found out what worked best for me. The hard part is to find out where your limits are. Yesterday I've managed to fist myself past the knuckles for the first time. It felt realy tight because my hand felt numb for a moment.
I say limits, because I am not that flexible and feel backpain the next day. It's some unnatural position for the muscles and have to use some force to get it in.
What is realy important is preparation. I wear my 2 inch and 2.5 inch buttplugs quiet some time before I start my fisting attempt.
Also some fisting lube is needed, next time I use a LOT more lube. I have the feeling it will slide in much easier when I lube it royally.
This week I will try again and let you guys know how my second time went like.