Are there any tricks to make "U brake" stronger?
The easy solution is usually better brake pads. If you're using $2 brake pads, or just old worn ones, you're not going to get very good braking. Spending $10 on a pair of decent pads can make a huge difference.

As well, make sure the brakes are set up properly, especially if you're using two finger (short) levers. With new cables, true wheels and minimum clearance between pads and rim you'll be able to put maximum force on the brakes. 

If everything is properly rigid you shouldn't see anything flex when you apply the brakes. If you do, maybe look at a reinforcing plate (called "brake bridges"). Many older U brakes already come with those. It's a U or C shaped plate than goes over the top of the brake to reinforce the pivots. They're also used with V brakes but shouldn't be necessary if the bike is built properly. Old mountain bikes and BMX's often just had U brake or V brake pivots added to a frame designed for calipers, and the tubes would bend or twist when the brakes were applied. That made the brakes no better than calipers in many cases.