The weather. I make sure it is pleasant, sunny and an inducement for folks to get out. Next is properly placed directional signage.

online marketing works the best.... on many sites, especially google+.... use the right keywords that a potential buyer would use TO FIND OPEN HOUSE EVENTS FOR THAT DAY.... I'd use: Open House May 1 Franklin MA....as an example.... that would bring up all of the open house events in franklin ma on that day...

I have a rider about my open house direction sign that has FREE sports schedules.  I also put up rider on Tuesday for open house on Saturday.

I never had a problem of getting people to the target. Anything goes is my answer to you if you are not getting the expected results.

I put up signs , use balloons on the signs and advertise online and offline.

Mailers to the neighborhood. 

Usually mailers and lots of online advertising.  Balloons & signs.

I wish I did

Putting it in the MLS and letting that sync to the online portals.  100% free and drives 99% of my traffic.

I just advertise them on CL and the schedule for the day fills up fairly quickly.

Signs.

Promotion -- offline and online especially mls since that is picked up in other forums.

Perhaps, most importantly:

The right attitude -- if you don't like open houses or don't find them worthwhile -- don't do them.

Promote on various on line sites, contact your SOI, do a mail out to neighbors.

My blog used to do well for these

We've been able to draw pretty big crowds by sending the neighbors post cards asking them to invite their friends.

I send 100 postcards to the neighbors and open house signs.