[fontgeek]

I guess I'm to old fashioned. I'm a firm believer in an actual written/printed invite with an RSVP included. If you include a pre stamped and addressed envelope for the RSVP. I makes it pretty tough for most to avoid.
In your invitation, include the theme, and the importance of knowing if, and how many people will attend. Maybe a costume contest or game that uses the costumed guests? Doing the invite itself in the theme can help get invitee's in the mood.
Suggestions for costumes may help too. Some people draw a blank if it's not a theme they came up with themselves.
I'm happy to help with the invite if you want it (the help, that is).

 

[fontgeek]

State in the invite that it is STRICLY an RSVP access only party. Explaining why often helps too. If you have a "phone tree", call the few key friends and make them understand the necessity/importance of the RSVP, and let/get them to do the calling and cajoling of the other guests. Once. you get the ball rolling it usually works without much hassle.

 

[fontgeek]

So much depends upon the personality/mentality of the guests. Some just come because it's a party (and something to do or drink), while others enjoy the spirit and camaraderie that comes with that kind of party. For many, especially if the party is in the middle of the week, they see the costume/theme side of things as too much labor/time when they know they have to get up and go to work/school in the morning, and many feel they've "Grown" out of the phase where they need or want to dress up. Oh, they still want the party, but they tend to take it as more of something catered strictly for THEIR enjoyment, rather than being part of it as an active participant. They kind of lose the idea that it's a chance for them to "escape" for a short period of time. In the end, you need to decide, as the party's host, whether those non-participating guests are worth the labor you put into your event. If not, don't invite them. If they miss a year, and ask why you didn't invite them, tell them the truth. You may find/make many converts to having better parties in the future.

 

[fontgeek]

I know it may make me seem old (fashioned), but often times, an actual written or printed invitation gets better results, even if you ask them, or offer them the option to RSVP via the internet.
Evites, no offense, are often taken to have the value seemingly put into them, namely zero.
Something in the physical realm (real life) tends to be perceived as having a greater value to the recipient because it's a tangible object that shows you cared enough about it to go through the effort and expense to create, address, and mail them. It also gives them, the recipients, a physical reminder of the event, including all of the pertinent information.