I saw one in a Middle School that I thought was really great. SOmeone had taken a picture of every teacher in the school reading a book of some kind from the library and blown it up in black and white. All you could see was heads and book (with titles) in the pictures and they were placed on a long board with the school colors behind them. Pretty unique and it can stay up for a long time since there isn't a holliday or strong theme in it.
I have (HAD) the same problem as you. Most ideas I've found are too young for my high school readers. However, recently when doing a search on amazon.com I found a wonderful bulletin board idea book for secondary students. It's called "Look Again! Appealing Bulletin Board Ideas for Secondary Students" by Judy Serritella. The ISBN # is 1586830538 if you would like to do a search. I've found many ideas and don't know where to start with them all. I sure hope this helps you. Let me know if you find it!