All I can tell you is what I just did - right, wrong or indifferent. All in all, I felt good about the results of my recent party but as most of us would say, I would do a couple of things different next time. Maybe you can benefit from that!
Plan ahead - give yourself time to plan a theme, advertising, get loads of awesome sponsors and find ways to make it fun. At least a month.
Pick a date - How long will it run??
Advertising - Make sure you get the word out EVERYWHERE!! Seriously...the more the merrier. Make or get yourself a button so people can put it on their blogs and spread the word.
Sponsors - Many people may have built up enough of a reputation that they have sponsors banging down their doors but as a newbie, I just flat out emailed any and all sponsors that I really liked, told them my plan and asked them if they would be interested in participating in return for some great free press. I was humbled by some of their responses. No one was ugly. You may have the most good luck with businesses run/owned by other Mommies as they really try to support one another.
Make it fun - Making it fun is really important. If you need some ideas, look around at other blogs and see what they have done. Remember that you want and need to make this benefit the sponsors as well as you so you need to make people need to click on the sponsor's site as a way to participate in some of that fun. Bottom line though, give people chances to enter at least once and possibly more.
If you have any specific questions I can answer, I'd be happy to try and I hope this helps. I cannot wait to see what you come up with! Let me know and I will help advertise it on my blog!