move ya head outta da way of der fist thats really all it is just having the nerve to wait til the punch is an inch away from ya face is the tricky bit
For some fighters slipping punches can be something they should avoid doing. Not that I mean you shouldn't move your head out of the way of a puch if you can (no ****) but simply placing more emphisis on good head movement and/or good basic defence rather than attempting to replicate the gracefulness of someone like Ali. First you need great reflexes and a good eye, not only to avoid the punch but to be able to quickly regain footing and balance to be in a position to avoid the next blow or to counter back. I'd suggest you first work on parrying and general defence with arms and gloves before going on to good footwork and balance. Once you've got that going on you can then start about focusing on slipping punches. Many fighters have done well by moving their heads without worrying about slipping a punch within that split second of it being thrown (think Joe Frazier) but if you believe you have what it takes to slip punches well AND it's worth putting in the time because it suits the way you fight then it's simply getting used to having punches thrown at you from different angels. You can do it while working the pads or whilst sparring. That said, some people could spend years trying to perfect the art and still not master it while others are just naturals.