The old solution from film SLR days was a 5X magnifying finder (with right-angle prism) that attached to the camera eye lens. I used to have one but tossed it along with my Pentax SLR years ago.
Focussing with these is easy, if you have a bright object - anything - in the field.
They are still around on eBay; for example this:
http://cgi.ebay.com/1-25-2-5x-Angle-...ht_3223wt_1167
The other trick is being able to lock the lens focus so it wont shift; most lenses don't. One solution is a piece of adhesive tape between the lens body and the focus ring, use this to nail it in place once you have focussed.