A very large number of tags on Stack Overflow don't have tag wikis at all. How can users be motivated with enough rep to write and edit them more often?
A great deal of users asking questions like: "Where can I find some references for X or Y?" or "How to do this most basic thing in Z?" could be simply shown the appropriate tag wiki. (Yeah, I know. It would've been better if they looked there in the first place, but many new users don't know that they exist).
I described my idea in a separate answer.
But, some of you will probably think of an even better or easier way to promote this behavior. So, let's hear it.
EDIT: OK, thanks to everyone for their participation in this discussion. We have some really great answers here, and it was next to impossible to choose the right one, partly because they are all good, and partly because of the tendency of a discussion not to have just one answer. So, I picked the two answers which seem to be supported by the largest number of users to accept and grant bounty, respectively. I hope some of these ideas will be used soon.