Well, undrentide said it best, but I just wanted to add something to his/her explanation of ~ておく. Since this is a construct used to indicate that you are doing something ahead of time, 保管しておく has the feeling that you are not only keeping that part of the ticket, but in fact keeping it in preparation for something else that is yet to come. In other words, there's sort of a "just-in-case" feeling behind it. I personally would be more inclined to keep something that said 保管しておいてください rather than something that simply said 保管してください--the latter seems to me that they are just asking because they're supposed to, while the former makes it seem that there is some reasoning behind their request.