MichaelB,
If you can't get more than 9.5V after several days you probably have a bad cell. If the cell sulfates and the plates are shorted out, you will not be able to recover it. Some have had luck with draining the acid, flushing and refilling with new acid, but at best this will leave you with a battery with less amperage than before. My best advice is to keep the battery charged with a good float charger (battery tender). I use a five gang battery tender and pull the batteries out of the bikes and hook them up on the bench. Watch the fluid level and top off with distilled water. For the BMW and the 850 Interstate, I use individual battery tenders as I leave the batteries in to be ready for that occasional nice winter day. The Deltran Battery Tender is a good product, but I'm sure there are others just as good.
Alternatively, a friend just buys a new Taiwanese battery for his pre-MkIII Commandos each spring. These can usually be bought for less than $30 and last all season.