As seen in Bridal Guide magazine
Q: How can we keep the guest list under control?
A: Your guest-list size is a key issue because it will largely affect the total cost of your wedding. In most cases, you, your fiance, and both fo your families will each have their own idea about who should be invited.
To get off on the right foot, have everyone involved submit a list of the people they'd like to have attend. They should rank the names in order of importance, starting at the top. Then you and your fiance can compare the lists and cross off any duplications. Add up the numbers that are left to determine how close you are to your estimated count and figure out how many more people you may need to eliminate (or, if you're lucky, who you can add). Then, have each of the parties involved cut an assigned number of names, starting from the bottom of his or her personal list. This way, both families are treated fairly, while you manage to keep control over the size of the guest list.
For more guest list advice, visit How to Keep Your Wedding Guest List Under Control.