Batch edits - how to move groups of your books from one shelf to the other.
Power searching tags on BookLikes (this looks super useful - it's an old post though, and BL might have tweaked their search since it was posted, so ymmv)
Reblogging - keeping the OP's link Super-touchy subject, I know. But this post shows you how to make sure the original poster always gets a credit link. Until the day reblogs are done in a way that makes more people happy, this is a really good tip; hell, it's a really good tip even if they do fix reblogging.
Notifications - telling the old ones from the new. This post solved something for me that's been driving me nuts! The images are out of date (BL has changed it's look since it was done) but it still works. Also, new notifications aren't highlighted in green anymore, they're just shaded darker than the old ones.
*New*
Adding BL linked book covers, titles and authors into the body of your post. - Ever wonder how people are able to easily link cover images and text links to books in the BL database? Here's how.