The ollie is easy once you have mastered the timing and drilled the movement into your muscle's memory, but the trick still needs your perfect balance on the skateboard, that or the outcome will not be desirable.
Your back foot should be as far back on the tail as stable balance will possibly allow, with your front foot shoulder's length apart from your back foot near the middle of the board a few inches away from the deck bolts. The more the back foot hangs off the edge the easier it is to pop the board off the ground. Body weight should be over the balls of your feet and centered in the middle of the board. This is for the best balance; it will keep the board straighter and can help in sticking the landing. Any improper foot placement can throw your weight off or tweak the board, screwing any chance there might be in a landing. The last thing is bend your knees. This does two things: it readies the knees for the upcoming jump, and it lowers the center of gravity adding a little more balance.
The next step is the hard part, to complete the trick successfully all these motions must come together simultaneously. You need to pop the board and jump using your back foot while dragging your front foot straightforward causing lift. The dragging of your foot should be with the side of your shoe until just past the bolts, subsequently this also raises the tail leveling the overall flight of the board.
The pop is a big part of adding height and control to the board. A good, firm pop will create the needed upward motion to get off the ground, making it so the front foot does not need to do as much dragging to obtain any measurable distance off the ground. The board, literally, is drug along with your front foot for the ride. Any extra foot movement can impart an undesirable change of direction onto the board, and because of the significant difference in mass between you and the board; you won't be able to change directions as fast to recover.
The extra foot movement is needed when doing other tricks, where the goal is to impart a slight tweak to the board from the combination of both feet to force rotation of different directions. An ollie is not like the other popular examples, like the kickflip or heelflip where the rider comes off the board and flips the board in a barrel-roll style. This trick is done by popping the board and flicking the front foot off either side causing the desired rotation. The heelflip uses the same technique but uses the heel of the forward foot. Other tricks use the combination of both feet working in perfect unison. The foot placement for these two tricks is also different from the Ollie, with the feet positioned closer to the edge and away from the center. It is important to keep all outside movements to a minimal on all tricks.
Now, let us quickly go over everything once again. Bend at the knees, pop the board firmly and jump with your back foot the instant the tail smacks the ground. Once done correctly, level out the board by dragging your front foot straightforward. Wait until you and your board return back to Earth. Bend your knees to absorb the impact and quickly shift a little bit of your weight to your front foot or the board will shoot out from underneath you.
Now try it while rolling if you have not already. Just remember momentum can be a tricky little bitch to take into account, and everything learned standing still could be adapted to the trick while moving but not easily. It is all about seeing the trick through and spotting the landing in your mind before even attempting it physically.