Here is a short, incomplete list of where you can park to ride Watchung:
1. The parking spaces on either side of Skytop near the picnic Pavilion.
2. Seeley's Pond parking lot.
3. The small gravel lot just east of there.
4. Any space around the circle that contains the playground.
5. The dirt lot on the east side of WR Tracy Drive north of Surprise Lake.
6. The large paved lot South of Glenside at the northern edge of the Reservation above the Deserted Village.
7. The unpaved lot east of WR Tracy just south of the traffic circle.
8. Trailside center where Coles Ave and Summit Lane meet.
9. The large paved lot between the turf soccer fields and the police training facility south of Glenside toward Summit.
Google maps has a pretty good trail map, but there are better (for example):
http://ucnj.org/wp-content/uploads/2014/06/Watchung-Trail-Map-020314.pdf