I think it depends on your relationship with you recommender. Sometimes just an email would do. However, I know I have been quite a pain in the whole process (adding and removing schools from my list). I've tried to come up with some gifts for my recommenders that are thought out. For example, I know one is obsessed with superhero movies, so I got that one a super hero book (lame, I know) and a thank-you card. I know a lot of students who get them chocolate or whatever too, and that is nice, I just figured to be a bit different. Still not breaking anyone's budget, and little bit more from the heart. There is plenty of cute chemistry trinkets on the shopping websites these days as well.