Don't know if its 100% possible to prevent drop eye. Theres too many unverified claims of drop eye causes floating around.
Here are some of the causes that I have heard of:
1. The arowanas unhealthy diet of fatty foods, namely feeder fish, causes fatty deposits behind the eye (seems like the most plausible reason)
2. An Arowana sees its own reflection in the bottom of bare tanks causing it to look down (This claim was disproved in a previous post)
3. The arowana was overfed while young
4. Poor water quality
5. Tank raised Arowana always get drop eye
6. It is caused by having food dropped in front of them
7. It is caused by having the arowana search the bottom of the tank for food
8. Having a bottom dwelling fish catches the arowanas attention forcing it to constantly look down.
9. It was caused by genetics
These are mainly the ones I keep hearing over and over again, though I think most of them are bull. Also young arowanas don't normally show signs of drop eye until its around 8-10 inches so it's not really possible to tell if you're doing a good job until its too late. Sorry if you didnt get the answer you were looking for, but I tried.
Btw, heres a link to a previous drop eye discussion:
http://www.monsterfishkeepers.com/forums/showthread.php?t=101149&highlight=drop+eye