how can i slip more effectively

Firstly each slip has it's own purpose, most of the time it is to set your defence into offence. A slip should be pretty much the same motion as throwing a straight left and right, combined with a slight bob. When slipping the jab to the outside, pretend that your throwing a straight left (without actually punching), combined with a bob. Now when you do this you will realise that you have to move your feet to remain balanced. To bob/weeve and slip effectively you have to have great footwork, and remember to put the same effort into your slips as you would your punches. Think of it as the start of your own flurry. Hope this helps, if you have more questions just ask as i've been trying to improve my head movement a lot in the past months and these tips helped me, although i can't really show you on here. If you can get your hands on the Jack Dempsey book he shows in detail bob and weeving, which will help anyone out along with anything you could want to know about punching.