Yeah, trimming is a continual battle this time of year, especially with the rain that we've had lately. In general, feel free to use loppers or hand pruning shears to prune offending foliage anytime. Try to prune entire branches at the stem rather than leaving sharp 'Punji' sticks that could impale the unwary. Toss any branches cut into the woods, away from the trail. Aggressively trim aggressive plants, like raspberries and grapevines, and less aggressively trim slow growth such as maples and oaks. If you're confident in your ability to identify invasive species (see http://web4.msue.msu.edu/mnfi/education/factsheets.cfm
), feel free to get rid of them completely.
It's a delicate balancing act to prune singletrack, making it rideable and approachable while still maintaining the singletrack 'character'. In general, trim branches but never entire trees. Trim to allow the passage of a bike and rider, but don't trim to improve lines, straighten curves, or otherwise change the line of a trail. At Lakeshore, we frequently transplant trees closer to the trail to help define lines, discourage bandit lines around choke points, and for aesthetic reasons. Usually these transplants are obvious - leave them be, even if you feel they are too close to the trail. We put them there for a reason.
I've found that the most satisfying technique for me is to ride a fast lap, and follow that up with a slow trimming lap carrying hand pruning shears in my pocket, stopping whenever I see something that needs attention. You can get nearly everything with hand shears, and it's so much easier than riding with loppers.