I love that you can have a piece of art painted by your sister - it's personal, it means something, it's not a beer poster
-- it can be a conversation piece.
I have a bunch of framed posters from trips I've been on-- one was an art exhibition that I saw in London, another is from a photography exhibition in Sydney - I like the pictures themselves, but I love them because they hold a special memory for me.
You don't want your apartment to look like it came out of a design magazine, you want it to look like and have the personality of the guys who live there.
Take a visit to stores like Pottery Barn or The Door Store or Restoration Hardware - or even some of the even more expesive stores - -see what you like - shelving, accessories, etc.. then recreate the look for less, at places like Target or Pier I imports or other stores. Just get the ideas from the design heavy stores.