Besides just totally going for it which is sadly what's quicker, I have an alternative. If your park has slide on's just go up to them from the side, and treat it like an ollie on. While you will not get the same experience with an actual ollie on this will help you develop the best angle for hitting one. I recommend back side board slides on ollie ons too, you wont slide off as easily and it has a less chance of ending horrifically. 50-50's are basic but you will slide off much easier when doing those.