Down climbing is always harder than climbing up, and more dangerous as well. Most serious injuries on mountains occur during the descent.
The technique to down climbing a boulder problem is to choose your route down before you climb up.
Before you ascend anything you should always be thinking about the descent and planning your escape route. Lots of times you can simply walk off the back of a boulder, or down climb an easy section. When you do down climb, make sure you are aware of where you holds are, and carefully plan out your beta for getting down. During your descent, go slow, and be extra cautious, and still with a spotter, your spotters can help you find holds with your feet as well.
It's not unusual to exert more energy on the down climb than on the way up. This is the nature of eccentric resistance (the un-flexing of muscles). The slow and controlled release of muscles under weight (eccentric training) is actually an effective exercise for breaking down and building muscle and strength, but it wears you out quickly.
It isn't unheard of to get off a boulder by down climbing a nearby tree, or even using a rope. My climbing club in university had a retired rope they tied into a rope ladder like this:
Rope Ladder Knot:
The nearest boulder was very close to a perfect cube with a flat top and steep vertical sides all around. The best way off was to either down climb to a lower ledge to jump off, or down climb the ladder, which was tossed over the top of the boulder and anchored to the ground or another person on the other side.