Equip a weapon and press RED to jump. Immediately upon jumping, change weapons. This will cause the jumping action to be repeated, so you can double jump. In this second jump, if you change weapon again, you will jump another time. You can repeat this as much as you want.
This lets Rico climb any building or gap, provided you start with a forward (not stationary) jump. If you don't manage to do the trick, rico will simply fall. If you do the trick too quickly Rico will either stay still or go horizontal. It is best to alternate between two weapons rather than to cycle along, and to have more than 2 weapons in case you change to unarmed or don't change weapons at all.
This trick allows you to get huge base jumps such as my best, 2129m (twice the achievement record). Very useful technique if you are stuck in the jungle, miles away from any road.