Hola! i've been told many times to relax when hitting the pads, bag or sparring. But often when i do these things, i feel relaxed, or i think i do. So how can i practice to relax more. I've heard that relaxing is pretty important, sooo.. kind of need advice hehe Peace!
you can start by using your hips and shoulder to throw the punch instead of the arm. Alot of 'stiff' guys hold too much tension in their arms when they aren't really the muscles doing all the work. Also don't worry about power, power will come when your technique and punch mechanics are good. Until then you need to worry about putting leather on those pads as fast and technically correct as possible.
Relax your whole body. Take you fighting stance ( your body should feel like strawberry jello) and just throw the punches. Thats how i did it and my trainer hasn´t complained since.