I was having a hard time thinking of a WFMW post until I read Not Trying for a Boy's post today.
What works for me when inviting people to an event is evite. I started using it for big events like baby showers. Now I use them for just about any get together. Saturday night Settlers of Catan game night...evite.
The evite is sent to everyone's email. They click on a link and RSVP. It is easy because you are already on the internet anyways. Everyone can see who is invited, who has RSVP and comments (i.e. I'm bringing chips). You can also see who has looked at it and NOT RSVPed! As the host I can email everyone, email those who have responded and email those who haven't responded. Since I can see who has and hasn't looked at their invitation I only call those who haven't looked at the invitation. I can also email people incessantly who have looked at the email and have NOT RSVPed.
That's what works for me. Check out other WFMW posts.