When I work on this at 24 hour gym they have Life Form(I think) equipment that is very effective to use for punching and knees. I wish I could recall the exact brand(I'll be at the gym tomorrow so I'll post the exact brand then) but you can use the ones for flyes for standing overhand punches and the one for curls for uppercuts. The machines are super smooth and you can simulate most of the punches you want to do. The best one is one that has two movable "arms" so you can stand in the middle, adjust the levels to what punch you want to work on and do 2 punch combos very smoothly and with resistance. You can also lower the arms down for ankle attachments for knees or do a punch/knee combo.
I'll post the equipment brand but if you have those in your gym they are easy to do, adjust and then gradually work up the resistance while maintaining good form to work on power.