Speaking from someone who has hair growing everywhere, seriously get a helping hand. Its virtually impossible to shave your arse or back by yourself properly without being a contortionist or leaving yourself with clumps or hair left behind or cutting yourself.
Seriously, Mangroomer. It's like using a backscratcher. It's a little tricky to get used to, but it does work. As long as your back isn't like a carpet, you can get a decent job. The trick is to go by feel. Like using a backscratcher, go up and down across the areas with hair. Use the feel of it to "scratch" the whole area. Then turn your back to the bathroom mirror and use a handheld mirror to see if you missed any spots.
Your arse is easy. If you can scratch your but, you can shave it.