How can we all encourage people to clean up their wish lists? The times I've peeked at a wish list to see if there is someone who wants a book I have, it seems they're all old posts.
I'm not sure we need to be proactive about this. If you want to offer someone a wishlist book, even if the wish is many years old, go ahead and offer; if they still want it - and some of my older wishes are still very much desired! - they''ll be delighted, and if they don't want it, or don't respond at all, you can offer the book to someone else or wild-release it in good conscience. [I do recommend including a time limit in any PMs to BCers you don't know well: "I saw name-of-book on your wishlist and would be glad to send it to you; please send your address within the next week if you want it."
There are occasional reminder posts in the Random Acts forum or elsewhere, about keeping one's wishlist up to date, but if the BCers in question aren't active on the forums and only receive occasional offers of books, there may not be much incentive on their part to tidy up.
And whenever you find the whole wishlist aspect getting a bit too complicated and/or frustrating, by all means, just wild-release the books. Just because somebody posted a wish for a title doesn't obligate anyone to offer to send it to them!
If you have a book you'd really like to send to someone who wants it, you can always post a RABCK offer in the "Random Acts" forum, or a bookring/ray offer in "Bookrings". That will attract people who follow the forums, and while it doesn't guarantee a safe journey with lots of prompt JEs, it does help...