If you haven't tried riding your mountain bike at night you are missing out my friend. Don't let early sunsets keep you off your bike and see old favorite trails in a new light, pun intended. Trails also see a lot less traffic at night. If you're typically getting your rides in on weekends during the day you're in for a pleasant surprise. Usually you practically have the place to yourself.
If you haven't ridden a trail before you may want to tag along with friends or take it easy your first lap. Lights can make it hard to judge the height of drops and the size of rocks. Start out with an easy and once you get more practice under your belt head hit up the gnarlier stuff.
A decent light is a MUST for night riding. New LED and battery technology is so incredibly far ahead of the old stuff. These new lights are way brighter, more powerful and longer lasting. At the very least you're going to want 350 lumens on your handlebar and a decent tail light so you can be seen. A huge upgrade is a helmet mounted light to extend your range of vision.
San Diego may be the land of never ending Summer, but once the sun goes down it can get downright chilly. Pack some full finger gloves and a lightweight jacket you can stow away in your CamelBak or bag once you get warmed up on the ride.
If you don't have a bell, get one. Announcing your presence to other riders is super important with reduced visibility. If you're in coyote central it'll help you avoid surprising them.