I've attended a flock of Startup Weekends - Austin, DC, Houston - and was one of the organizers of the last one here in Austin.
The mix of attendees & skill sets is both the hardest and most important thing. The approach we took was multi-faceted:
- we announced it quietly to the tech groups first - mobile dev, rails, php, python, etc - with an embargo on tweeting it;
- once we started spreading the word further, we made a very small number of non-technical/non-designer tickets available;
- once those were sold out, we put the entire event into waitlist mode.. from then on, to buy a ticket, you had to send us a Linked In profile to prove your background.
And once we opened it back up to business people, we wanted to get the most motivated, so we asked everyone on the list to make a 1 minute video pitch on why they should get a ticket.. and then tweet it to us with the event hashtag. Most didn't bother making a video, so the decision was simple.
The result was a good mix of people and the non-technical people were very motivated and (probably) felt lucky to be there.
My 0.02.