Definitely give away as many free washes as possible to charities, fundraisers, school sports teams etc. We often give gift cards to silent auctions or giveaways in our community, because every new face is a new possible customer.
When it comes to fundraising for schools and sports teams, we sell gift cards worth $20 to the students for $10, and allow them to go and resell them to parents and the community for the whole $20 price. This allows them to make $10 per gift card sold, covers our water, power, and chemical, and again gets new faces to come through our wash.
We also pay one of our employees an extra 2-3 hours per week to post on facebook and instagram, and to engage with other local business' on social media. I follow all of our pages to make sure that the posts are consistent and appropriate, and to comment on the posts that they put together to boost engagement. (The more comments and likes, the more people get shown the post.)