If a haunted house in the woods has no teenagers with a broken down car there to be scared by it, is it really haunted? The scariest haunted attraction in the universe will never scare a single soul if nobody knows that it exists.
People telling other people about things is the biggest, most common and most ancient method of free advertising known to humankind (as well as ghouls, goblins and other various undead). Tell your friends. Tell your enemies. Tell your enemies' friends. Then tell your enemies' friends' mothers. Tell everybody.
Social Media is a newer form of "Word of Mouth". It's like holding a bullhorn that's loud enough to reach across the Earth. (Of course, only those who follow your accounts will see those words.) Your haunted attraction should have a Facebook business page, as well as accounts with Twitter, Instagram, Youtube, and whatever new one comes out tomorrow right after this page is published.
Facebook: If you don't already have a personal facebook account, sign up for one. Once you do have a personal account, you'll want to create a business page for your haunt. Your personal page is for friends, and your business page is for people who like your haunted attraction. However, your personal "friend" account can drive business to the business account through your personal interactions. Just remember to not make your personal account overwhelmingly about advertising the haunt, or you'll lose friends.
Twitter: Even if you already have a personal twitter account, you'll want to sign up for one for your haunt.
Instagram: Get an Instagram account here. Instagram is mainly used through a phone app, as the main purpose of the service is to post photos and short videos taken on your phone, which you upload directly afterward, along with whatever text you want. For example, you can post a photo of work in progress on the haunt, or scared patrons coming out of the exit. You should also make sure that your instagram account is set to automatically post your instagram photos to your haunt's facebook and twitter accounts as well. One great use of instagram is to get photos of guests (while they're being scared?) and ask them to tag themselves and share the photos on Facebook.
Youtube: You can sign your haunt up for a youtube account here. Youtube is the world's largest video sharing social platform. Use youtube to upload promotional videos, clips of the haunt being built, guests being scared, whatever you'd like. Take advantage of it.
Many social media platforms allow you to place paid ads. The one that I've found to work best is Facebook Ads.
Facebook has a decent and relatively low-priced option for advertising. You can spend $5 per day, $10 per day, whatever you want. Basically, you set up an ad to run for (x) number of dollars per day, then set how long you want the ad to run. You can use targeting to select people who are into haunted houses, Halloween, etc., within a close radius to where your haunt is located. Be sure to use an eye catching photo with your ad. You can test the waters by running an ad for just a few days at first to see what kind of results you get. Check out facebook ads here.
"Boosting" a facebook post is another way of using facebook ads, and it's actually simpler than going through their traditional ad system. Just post your ad as a status update on your attraction's facebook business page, click the "boost post" button underneath, and you will be asked to fill in some targeting details in a pop-up window.
Have paper flyers printed and post them up at Halloween stores, costume shops, and any other place you can think of where haunted house fans might be. Think of potential fans as well. A lot of people who aren't already fans of haunted houses will become fans in the future. You could give them that introduction. To find local printers, use a search engine (yahoo, google, bing) to search a term like "flyer printing near (your city and state)".
A street team is a group of people who distribute your flyers and talk about your haunted attraction online on various websites. These can be you and your haunt volunteers/employees, or completely different people altogether, if you choose. For physical flyering, have your street team each take an assigned area and deliver them to any shop that will allow you to place them next to their register - Halloween shops, convenience stores, gas stations, book stores, music stores, clubs, etc. For online street team activities, assign your team members to post about the haunted attraction on message boards, facebook groups, 4chan and reddit boards, etc. Be sure to not let the advertising seem *too much* like straight up advertising, or the posting might end up being considered spam.
Radio commercials are one of the biggest ways that haunted houses advertise. I regularly hear radio ads for multiple haunted attractions in my area starting in September, all the way through October. A Guinness record winning haunt near me uses radio ads heavily. Contact your local radio stations that Halloween and Haunted House fans might be listening to. Hint: It's probably the ones who have a demographic of 18 to 34 year olds.
Advertising on highway billboards, the sides of busses, etc. seems to be another one that the big guys are doing a lot of recently. I once saw a full size haunted house ad across the side of a bus while I was simultaneously hearing a radio commercial from the same haunt. Now *that* got my attention. Search the terms "billboard advertising (your city and state)" to find which companies do billboarding in your area.
Make sure you have large signs in front of your haunt, and by the road. This will attract people who just happen to be driving, walking, skipping or skateboarding by who would've never discovered your haunted attraction through any other means. Vinyl banners are a weather-safe, low priced way to go. But, you can use whatever material you want (as long as it's allowed by local ordinances).