I remember when i first started learning boardslides, someone told me to position myself like I was petting a dog. Lots of people have the problem of leaning too far back when they boardslide, and if you bend your knees with your hands out just in front of you, you automatically assume a decent boardsliding position. Literally as soon as I tried that out, I got boardslides down immediately.
After you get those, frontsides and back lips will be a lot easier.