I've been sliding since 2002; so here's some things I've learned over the years:
Stretch & warm-up: Do this anytime you plan to slide (whether it be practicing or performing at the haunt); this helps avoid pulled muscles, aches, and sprains. Also, do some cool-down stretches at the end of the night to avoid cramping.
Practice: train your body to get comfortable with falling (sliding is essentially falling with style and tricks); this is actually really hard for many people. Create mini obstacle courses to perfect your stopping ability and maneuverability (you don't want to end up sliding into a customer). If you know of other sliders, practice with them and work on double-tagging tricks and synchronized moves.
Gear: Every slider will give you a different response for their favorite brand; I prefer thick industrial kneepads over the thinner skater kneepads (I find they cushion my knees better and almost never shift), but that is all a matter of preference. Steel-toe boots/shoes are a must (you will destroy anything that isn't). There are spark gloves available for sale and tutorials online for making your own.