So Zara's bat mitzvah is next spring and I'm busy making arrangements. The caterer asked me to estimate the number of guests - adults and children.
What did I do? I put together a spreadsheet, of course. Each row has a potential invitee or invitee family. The columns are: Name, # of Adults in Party, # of Children in Party, Category of Guest (e.g. my family, S's family, Zara's friends, etc) and my estimate of the probability they'll be there. Then, of course, I had it multiply that probability by the number of guests in each age group to get a total.
The books say to take your guest list and figure 80% come. I think my way is better, if perhaps excessively geeky.