1. Make your assets 'stand out' - great app icon, relevant / catchy name, great description with some user reviews.
2. Get your keywords right. Look at keywords used by similar apps (yourmechanic ..etc), google keyword tool ..etc.
3. Create a simple, elegant landing page for your app and put high impact keywords there as well (in one of my apps we got more downloads from google searches than app store searches).
4. Submit your app to app review sites / bloggers using a concise, creative, compelling pitch. Sample list below:
5. Keep your app free and try to get users to tell their friends after they see some value. E.g, put a "spread the word" button before the 2nd maintenance request which will pick say 10 most commonly contacted people from their address book and sends a non-salesly text message. Do some A/B testing here to figure out what works best.
6. Look at "pay for performance PR" to see if someone can get press coverage without a lot of spend. One example below, search google for more:
7. Reach out to press reporters on twitter and pitch to them nicely (i.e figure out if they write about car maintenance, appreciate their articles, and then insert your app as a relevant solution to what they talked about). Google "finding reporters on twitter" for lists, I know there are twitter PR lists.
8. Signup at http://www.helpareporter.com/ (free) and keep watching for reporter inquiries related to the maintenance industry, and help them out - they typically ask for stats, experiences, interviews ..etc. You can get free PR coverage with that.
9. Try distributing flyers near car dealerships, maintenance centers, repair shops ..etc. Incentivize mechanics to spread the word about you.
10. Look at other partnership channels where your target demographic goes for car related services - not easy but one partnership could change the game for you.
Some 'paid app marketing' services you may want to look at: fiksu.com, facebook app install, twitter app install
Rumor is that if you're a free app and get 35-45K downloads per day you'll make it to 'top 25' which gets you the visibility to get further ahead - if your app, the 'first 5 minute experience', value prop is great.