As you approach the obstacle, let off the front brake. Your weight should be balanced on the pedals: if you're "hanging on" (pulling back on the handlebars), you need to let your weight come forward a bit until you're neither pushing or pulling on the handlebars. Your butt needs to be OFF the seat, and your weight needs to be CENTERED on the bike. Do not, repeat, do not roll into the tire trap with weight on your hands.
As you near the tire-trap, let off the rear brake as well. Just before the obstacle, rock your weight back a bit to load the rear tire, then pull upward slightly on the handlebars. As the front tire reaches the top of the obstacle, release the rear brake completely and quickly compress the pedals (like you're doing a little bunny hop) as you rock forward. This unloads the back tire. As the rear wheel hits and the bike bucks, keep your body straight while the bike rotates underneath you. As the rear tire clears the obstacle, make sure your weight is off the front end, then reapply the brakes.
If you've had practice at front wheel-stands, you can put your weight on the handlebars, then pull the rear wheel up with your feet. Be sure your body doesn't rotate along with the bike, or your weight will stay on the handlebars after clearing the obstacle.