I've heard them all & do my best to cover my bases.
1. post invite or send out invites at least a month before
2. have all the information- date, time, location, directions, email address, & 2 phone #'s to call, on the invite
3. all ages welcome
4. lights on "Chicken Clause" for the easily scared adults & children

I still get from time to time:
I forgot
Was the party this Saturday?
We got lost & it was too late to come by the time get got turned around
Couldn't decide what to bring
No costume
It's too cold outside to go
Yes, yes, yes... I'm definately coming to your party
Halloween is against my religion & I can't come

Actually had one person show up the following Saturday at the right time... oops