Single player replay making is good practice for multiplayer. You'll develop more understanding about what's effective in terms of producing power, and you'll be more familiar with what options you have given your position and momentum than you would with MP experience alone.
As a corollary to number 3, using all of your joints to power hits has the added benefit of setting up another hit, like compressing a spring, using (contract left pec, extend right pec, right rotate chest) or (extend left pec, contract right pec, left rotate chest) combinations to generate most of your power. Usually, you can alternate your hips to supplement this.
If you think of your hands and feet as the corners of a rectangle, a full powered strike with one point will set up power for any point with a connecting edge. So a right hand punch will set up a left punch or right kick. This is very important in MP, where you don't always have a chance to get grounded and change your momentum.
If you want your strikes to be heavy, make your body as big as possible (i.e. spread your arms out, keep your knees straight, etc. along the axis of your strike) at the moment of impact. It will increase the amount of torque needed to rotate you. This makes the difference between just snapping someone's arm off and having it float there, and smashing it and sending it shooting towards the ground for a fast DQ.