This is the second time that I have quoted this book today.
According to Brew Chem 101 by Lee W. Janson acetaldehyde can be avoided by:
Using good quality yeast, especially when brewing high-alcohol beer styles
Allow beer to ferment and age for the proper amount of time
Eliminate bacterial infections by maintaining sterile brewing techniques
The book explains that acetylaldehyde is made during fermentation, then processed into ethanol. If the yeast is too weak for the style of beer being made, it can poop out before this conversion is complete.
It says that acetyladehyde can also be present in young beer. Ethanol can also be oxidized back to acetyladehyde and acetic acid. This is interesting to me since it seems there is a window where the level is in tolerable range.
The book also says that acetyladehyde can be caused by bacterial infection.
Sorry for the long reply, perhaps I should wander into the drunken Rumblings forum.