I just wrote the longest most wonderful answer and it disappeared!!! 
So I'll simply say now, if I wanted to become known for having the greatest, most successful Open Houses, I would hire a popular Event Planner to learn all the tricks of their business.  Surely, planning society & charity events must have some overlap to what you're trying to do?  It is all a numbers game so how best to get the largest numbers in the door?  You're welcome

I love open houses and find that the more I prepare the more success I see at them.

• Advertise a week ahead.
• Appointment only, no walk ins who have not called.
• Set the showings 20 minutes apart.
• Set up a small desk near the entrance, with a stack of applications.
• Use the vacant time from those who do not show to call and schedule those who called and left message but you may have missed calling back earlier.
• We generally have several opens simultaneously, if they don't like this one we send them to the next.
• Always a busy day and results in a nice stack of applicants.

Lise, I just love Susan Emo 's answer.  That really works well for a very high end home, and it is by invitation only.  One of our clients did that and included a fund raiser in conjunction with the local chamber of commerce.  He invited the guests.  A

Having the right attitude is important --- there's nothing worse than seeing an agent who would rather be somewhere else at an open house(if you don't like this type of marketing, don't do it).   

Lots of directional signs can help drive traffic.

I present a nice product with little or NO noticeable issues. First impression standards are put into place. When you turn the corner to approach my listing, you see this followed by a smooth showing

I mail out 100 postcards to the neighborhood. Then, I place it on Facebook. Finally, I put out 10 signs with arrows leading to the Open House.

Lise Howe Facebook Ads and landing pages to register online for open house have helped me a lot.

Call me when you get a chance - if you'd like to discuss more about it.

Please note that not doing open houses does not equate to "I am too important" ...  there are a multitude of other reasons that have nothing to do with me 

Make sure the sign out front of the house is easily visible.  Sounds so obvious, but for a driver in a car trying to see which house when the street is lined with cars and the For Sale sign is lower than the line of sight, well they are likely to drive by and keep on driving.  Put balloons on it if you need to, or tie a big yellow ribbon around ... whatever.  Make the house grab attention, in a nice way.

I start days in advance asking the seller to help me promote on social media. I put a sign rider up several days before stating the day and time the house will be open. The day before I put out the pointers and directional signs. I have done Facebook ads and sent invites to those in the neighborhood. These are just a few.

I have not found a magic bullet Lise Howe but the closest I can get to that is extreme signage. Cover the neighborhood with arrow signs.

Bob Crane gave you most of my list. I also enter the new contacts into my CRM as they register and program it for a follow up email.

I don't do them because they don't work in my market, but I will share something a bunch of office and a well known lender tried many years ago. Everyone got together. I two page spread was taken out in a local paper with maps to the different open houses. Signs and balloons were put up all over the county. Gas cards were randomly drawn and awarded to the visitors. 

I had two people show up to my open house. One was the neighbor next door, and the other was the neighbor down the street. They were both just curious and didn't need my services. 

But I liked the idea of the marketing campaign that several offices joined in on. 

I'm not too important, I just don't get traffic so I don't do them

I do all social medias, 15+ signs and flags, fliers, if it is sfr, I leave invitations to OH;  bring comps and school info, watter bottles ( and ice), and some welcome signs, so people sign in. 

Sign blast.

Make sure the house is a Pokemon Go! stop.

MLS announcement

Active Rain blog

Facebook promoted post

Lots and lots of direction signs.

Plenty of directional signs and call all the neighbors.

If signs are allowed the previous day or even Friday evening when people drive home from work. They can plan the weekend.

Advertising in local media and online, emails to other agents who may be showing properties that you will be open, signage and anything like balloons that will get the attention of potential home buyers driving by.

marketing online and offline..... signage.... they'll see the signs without balloons....

An open house is only successful if potential buyers know about, so i promote, market and market some more

 I use the local dues paper and the MLS...

A great little post on MRIS - that's what winds up on the Washington Post Open House online section, which is what everyone seems to use.  

Then come the bread crumbs - lots of arrows. 

Is it for buyers or brokers?

• I have hired teenagers to wash everyones car that showed up
• I have had theme's
• I have had games and prizes
• I write posts about it constantly the week of the open
• I put a ton of signs on every street in the neighborhood leading to the open
• This week I am parking my car and attaching open house signs to the side and leaving at the subdivision entry.

Every sign we have put up in this neighborhood was stolen within hours. Hopefully my car won't be...

Mine are on Sundays.  I get them into the MLS by Wednesday.  That puts them on Realtor and Redfin (where most people are looking).  I manually put them on Z&T.  I run facebook ads starting Friday evening until about an hour or two before the OH starts.  Still playing with my targeting and geographic areas.

Kathleen Daniels nailed this one!

I seldom do opens

Lot of online advertisement such as MLS, Zillow Trulia and my webmaster that promotes online to many outlets.  Great sign placement is key though.