It's not the most glamorous job in the library, but weeding the collection is very important. I admit, I tend to be a bit lax when it comes to weeding, and I usually only do it when one of the pages is at me, practically in tears, because the shelves are too full and she can no longer fit even one more book on them.
I've attended a few conferences where weeding has been a topic of discussion, and I've learned a lot from other librarians on the topic. I've mixed that knowledge, and just my own bit of insight, and come up with a way that I think works well for weeding the YA section at my library.
First of all, there are some books that are just never going to get weeded while I am in control. I don't care how dated the cover of Losing Joe's Place is, I loved that book when I was in middle school and as long as the pages are still sticking in the book, I refuse to remove it from my shelves. Also, while not many teens are all hip on the Dumas work of art The Count of Monte Cristo they are going to have to pry that book out of my cold dead hands to remove it from my YA section.
That said, here is my advice when it comes to weeding in the library. First of all, you have to know your section. I run the teen section, so most of my weeding advice has been developed for a YA section. Teens have the attention span of a gnat. Honestly, if something is blinking, swearing, screaming out music, or flashing them, they probably don't care about it. So, if a book hasn't checked out in 3-5 years from a YA section, then it needs to GTFO, or have a really good reason to be on the shelf. A few years ago I found a book in the YA section that hadn't checked out in my entire LIFETIME. Yes, the last time it had checked out was pre-1979. Just today I pulled one book from the shelf that hadn't checked out since I've been able to drive. Yeah, that stuff needs to go.
Second, while teens aren't as bad as us old folks, and their eyesight probably isn't failing them as quickly (although they are increasingly losing their hearing) as it is us more mature individuals, teens are not going to take the time to squint and barely be able to make out the title on a spine that is sun bleached/bent all to hell/dirty/has cloudy laminate/any other thing that is obscuring the title. So, if you as a librarian can not clearly read the title, trust me a patron isn't going to even bother with that book.
Third, and this is a big one for me, the smell test. Seriously, if I open a book and it smells like it's been in someone's grandma's basement for a few decades, I take that crap out of my collection. Now, I may order a new, and not so smelly, copy, but I am not going to force my patrons to put up with someone that makes me feel like I am going to have an asthma & allergy attack.
Finally, and this should be obvious, but multiple copies! Yes, Twilight is the haps now, and Harry Potter was HUGE a while ago. But, if you have a book and there is not a HUGE demand for that title, and it is not part of a school required reading list, there really isn't a reason to have more than a couple of copies on hand at anytime. Especially if your library is part of any sort of group that can inter-library loan.
Remember, shelf space is important. Overly crowded shelves are actually a deterrent to patrons. So, get rid of those old, dusty, non-circulating, smelly, hard to read, and multiple copied, books, and give the other books room to breathe!