One way to do it, and the way I used to take for quite a while, would be to get on Route 44 heading east. Then, in Millbrook, take Route 82 south, and then take Route 343 east until you reach Route 22. This will deposit you right outside Dover Plains.
Another way of doing it is to take Route 55 east and then get on Dutchess County Route 21 (Bruzgul Road) east until you reach Route 22. You'll end up in Wingdale, and then you can take 22 north for a few minutes until you reach Dover Plains. This is more or less the way I get to Dover now, mainly because I think it's a little shorter mileage-wise, so I'd say this is the best way.
