Skills used to decay if you did not use them. That was taken out of the game sometime in late 2003 with the introduction of skill locks and arrows. Not sure if Siege kept skill decay? If you have skill locks on Siege, then I'm going to say they got rid of it there too.

Here's the old list of skills in order of decay rates (fastest to slowest).

http://www.uocraft.com/skill_decay.php

Many will disagree, but personally I find that if I'm at the skill cap and set a skill to decrease, it will decrease FASTER if the skill it is taking from is alchemy (the fastest decaying skill). I'm convinced that the skill decay rates never got taken out of the game (maybe mistakenly), even though the automatic decay was turned off. I've verified it for myself many times, as recently as last month.