When you start planning your wedding I strongly recommend you make three lists (the first three of many).
1. My not negotiables - what must be at your wedding, not negotiable. This can be anything from a a decor item to the kind of venue you select.
2. I really want - this list are the things you really would like to have at your wedding but willing to compromise on how it is presented/the look and such like.
3. I would like to have but it is okay if I do not have it at my wedding.
These three lists will help you when it comes to allocating expenses and keep focus when making difficult decisions on what to include and what to exclude. If the item is a not negotiable item and costs more than you originally budgeted for, it is okay but understand then that you will be taking money away from another item which may fall onto one of the other lists.