1. You should place your palms flat on your work surface, but if your forearms aren't level with the surface, you need to adjust your chair up or down until they are. If your chair isn't adjustable, you can effectively raise your seat by sitting on a folded towel, on a cushion, or a pillow. If your chair is too high but can't adjust downward, you're out of luck and need to either purchase a new Herman Miller Aeron or find a way of raising your workstation up to the proper height.
2. Please check your feet: Do they touch the ground, or are they left to swing now that your chair is adjusted to the proper height? If your feet don't touch the ground, place a phone book underneath them as a footrest.
3. You should also roll up a towel and place it in the small of your back to help maintain a natural curve in your spine. If there's a large gap between your back and the backrest of the chair when you sit up straight, you may use a roll of paper towels to support your back instead.
4. Last but not least, remember to sit up straight, with both feet flat on the floor or your footrest: Even the Herman Miller Aeron will not help you if you insist on slouching.