I used to be a planter when I started using an actual 4-step or 5-step approach and I didn't know how to slide. Planting your slide foot can really do some nasty stuff to your knee; while I didn't get injured before switching to a slide (I'm glad I avoided that), I was noticing the bowlers with better fundamentals and they pretty much all slide.
What you can do to practice this (I practice my form kind of like this) is: find a room or a hallway (make sure you have enough room to move around) where you have a hard wood floor and put a pair of socks on. Pretend you're bowling (without the ball and shoes and all that jazz) and make a pretend approach with steps, arm swing, follow through, etc. On your last step, slide your foot forward. Because socks slide more easily, they are useful for helping you practice a slide.
When you take to the lanes, try the technique you practiced and you will notice a major difference in your leverage and balance. You will also notice how much less stress you put on your hip and knee; soon, you won't have to wear a knee brace! How liberating that will be! I used to get "bowler's hip" a few hours after I got home from bowling when I planted my foot, but after I started sliding on my last step, I don't get that anymore (or nearly as much).
HG: 193 (?)
HS: 516 (143-183-190) (?)
Bowled regularly in high school, am on hiatus for college.