I don't have experience with citizen patrols on bike trails but some experience with citizen patrols in fairly high crime neighborhoods. One of the techniques they used was to congretate on corners and other spots that had become sites for drug dealing and other criminal activities. Backed up by roaming police patrols the citizen presence encouraged other residents to come out and sent a strong signal that the law abiding citizens were not just going to walk away from crime ridden areas.
Your citizen patrol might also help by looking at physical improvements to make the trail safer (trimming back trees and bushes that afford hiding places, limiting trail access from spots that have a high incidence of muggings and assaults [new fences, barriers or even planting briar patches], cleaning up glass or other debris as mentioned, etc.) Such activities send a strong signal that people are watching and active in protecting the trail. They also offer an opportunity for trail organizations to interact and make allies with community groups along the trail who have a shared interest in cutting crime.