Well, IMO, if you're trying to ollie a longboard, you're doing it wrong. Now if you're trying to ollie a regualr board, that's a different story. So, since the title says longboarding help, let's start there. Sliding takes time and practice. It also takes a wheel that is a stiffer durometer (hardness).
Start small. While cruising around, shift your weight slightly forward, then kick out your back foot and try to bring the board into a horizontal postion under you. As you're doing this, you'll likely find yourself wanting to shift your weight back over the tail. This is normal, and for learning purposes, it won't hurt you. But, what you'll wanna do ultimately is get comfortable with keeping your weight actually fairly centered on the board. So basically, you start with it a little over the nose, then shift it to the center as the tail comes out. Then it's just a matter of getting it balanced.
Once you get good with small slides, you can pick up some gloves and learn to really lean it back and throw it out. Then you can bomb big hills and slide for days. I hope my basic description helps out, and I'm sure others will point out the things I may have missed. Now....onto ollies.
Take your regular skate, and stand on it in you riding position. Now, move your rear foot to the tail so that your front half of your foot is on the tail, and scoot your front foot back from the nose a few inches. Now, before you ever actaully ollie, just practice pushing down on the tail, then rolling and sliding you front foot up to the nose. Do this a lot. It will help you learn the motion needed to ollie. Then it's just a matter of applying the pressure with the back foot and sliding the front foot up while in air. Basically, just do a one footed jump while pulling the front foot up. It sounds complicated, but once you practice it, you'll learn that it's not so bad. Just practice.....a lot. Once you can ollie, it opens up the whole world of skateboarding for you.