ajohnson is correct in that I look at many factors several of which he hit on the head. One that he mentioned does happen often, where I contact a member and for whatever reason can't accept the prize. So most of the time I have to ask several members before I get to someone who not only needs it but can use it. That is also why the winner sometimes isn't chosen until later in the month
I do try to have the winner selected before making the announcement, so that I don't have to keep making updates to the articles and forum threads. That happened in the past where I put someone in the announcement who found out after the fact that they couldn't use it. I felt it was only fair to give that member another prize in a future month. Lo and behold, another member noticed the winner's name twice in a given year and sent a very direct note to me questioning the process.
All of this happens behind the scenes, and it's something that I usually don't share. It's just part of running the site.
Another thought... Since we work very hard at coming up with these prizes, we don't want them to go to waste. So I ask bluntly that if the person can't use it (or thinks he might not be able to) then let me give it to someone else. I'd hate for a prize to sit on the shelf for a year and eventually not be used.
There is a method to my madness. So although it may seem random, I feel the eventual outcomes are more than fair.
Case in point... El33tsamurai. He was not the first member chosen this month. But in the end, he was the memebr with the most posts who had yet to ever win and who could also utilize the prize.
Hope that helps give a glimpse into how it's done.