As another poster said, no matter how you word it on the invitation, it will not change things. People will fail to come when they said they will, some will show up last minute with little to no warning. I feel like people don't want to say no if pushed, but that doesn't mean they'll come, so I think the best thing I've found out there is Evite. People are simply online more often than they think to post something in the mail or make a phone call, so you have more chance of getting a response that way...some could say to themselves "let me check with my husband, or calendar" and then forget about it, but most check email all the time....so more likely to get a response. Also, if the answer is no, it's easier for some to click "no" than have to come up with an excuse to your face. (Pathetic, but true). I would not include anyone that said "no" and 3 days before the party phone the ones that said "yes" or "maybe" on Evite. Another thing I like about Evite is that it has you answer with HOW MANY adults/kids will be coming with you (an opportunity to be warned about siblings or spouses, so you have enough to drink and all). Before I found this site, we had a near-catastrophe for my son's third birthday. We were taking him to see Walking With Dinosaurs (very awesome, but $250 for the 3 of us). We thought that'd be enough, but he'd been to a lot of parties in the recent months and knew that birthdays have parties (oh no).....he asked for a party and since he's my first child and this was the first time he'd asked for that, I was keen to oblige. I asked him what a birthday party is, what does he want, and he said "balloons, cake, friends--"My OWN friends", he said. I invited all the kids that we'd attended parties for and some RSVP'd no (busy weekend, short notice) but we had a "yes" for 5 kids. I thought that was plenty for a 3 year old, and I made arrangements to have the party at a great park nearby, spent a couple weeks arranging games, decorations, a pinata, made dips, hotdogs, homemade icecream, and cupcakes for all, and ONLY ONE girl showed up! Fortunately, I had 2 friends of mine who have known my son since he was born (but have no kids) come also. It was saved by the fact that the party was at a park so there was the playground, we had our food at picnic tables, and ran around kicking the soccer ball that is always in the trunk of my car. We opened the guest's gift at the park, they played on the playground together, and then we went home with my 2 friends and opened their gifts at home and continued to visit and play some. That would have been HORRIBLE if he were older and understood what happened (fortunately, I didn't tell him who I thought was coming.....I simply said "your party is at the park" and was leaving it to be a surprise). My heart would break if he were old enough to realize he had been stood up...People can be incredibly rude. I'm not sure what I'm going to do next year, but I know that I will only invite people I'm close enough to be honest and frank with. Quality of guests over quantity, for sure!
And a sidenote: my 3 year old already knows the rules of ettiquette better than many adults I encounter! At the end of the party, ON HIS OWN, he stood up on the bench so he could look the adults in the eye and said "Thank you, come my party". We also wrote thank you cards for each guest and gift and mailed them. (I wrote a message on one side of the card, and he scribbled something on the other side, and I helped him write his name). It's so important to show thankfulness and have BASIC manners!!!