I need help with ollies on my skateboard?

Hi, I have been skating for a couple of years but really never got into doing tricks. Until now. Because with all the curbs everywhere it's a pain in the *** just to stop just to pick up my skateboard and then get back on agin. So I've been practicing ollies. My friend has been showing me how but whenever I try, my back foot always slides off the back. I've been doing it perfect in the gr.(Don't tell me that practicing in the gr is bad because I know and I have been using a very old walmart board in the gr.) But like I said, my back foot keeps on sliding off. So if anyone can please give me some good tips, I would appreciate it very much.