Shave your Own Back
To me the key issue here is not actually doing the shaving, but seeing what needs to be shaved. I have used an electrical rotary razor with a trimmer section, and I've been content using the trimmer section.
However as indicated in this answer, it could be well advised investing in a woman's electric wet/dry shaver, 3 Blade Foil Shaver. One of the blades is a trimmer that cuts longer hairs, the other two cut them short. These could possible have a better handle on hair attached to a chest or the back.
I haven't been able to use a manual razor on my back, and don't foresee how it can practically be done. Using an electrical razor gives you so much more leverage when it comes to reaching those hard to get places.
I tried different setups with mirrors, and the one working best for me was to stand with my back against a wall mounted mirror, and then have another mirror, either hand held or fixed in a tripod. In one case I had the luck of using a mirror inside the cupboard reflecting onto the wall mounted mirror.
Either way I look into the small mirror to see the reflected image in the wall mounted mirror. The point being that it is easier to move/manouver the small mirror to see the correct section of your back, rather than first looking in the largest mirror and then finding the sector you are shaving.
Alternative: Using camera phone?
I haven't tried this my self, but you could possibly use a camera phone, web cam, or video camera which has a live feed, and then watch your back on a large screen whilst shaving.
When shaving take your time and let the razor do it works, just concentrate on moving it somewhat regularly up, down, sideways, which ever way you need to in order to cover the back.
Most people does actually have the dexterity to reach most part of the back when using both hands, and shift positions from over the shoulder to going around and up the back. And this is easier if holding an electrical razor which gives you that little extra in length to reach all over.