Many of the slipe and slides come with a sprinkler system that enables you to connect a garden hose to the slip and slide. The sprinklers then provide the slide with a constant stream of water to keep it slippery.
Having said that, some people squirt dish soap down the slide to make it even more slippery. It can be a fun way to achieve more speed if you're finding that the slide isn't slippery enough with just the water sprinklers.
You could also have a hose set up at the top of the slip to provide it with more water for added slipperiness if you feel like the sprinklers aren't doing the trick.