I found using forums is a great way to promote a game or app.
You can promote your app or game for free here:
Android Games & Apps Forum
Free forum : Android free games and apps
Android Games | Android Forums
Actually you can launch and promote your app for Free with cross-promotion! It’s a nice way to become more popular before monetizing, or even while you are monetizing. It will increase your fill rate and get you more downloads. Check out this worldwide community called Tappx. It’s completely free and it works on credits, when someone promotes their app on your app, you get credits that you can use to advertise your own app. If you don’t have banners, they have some that you can use for free too. So all in all, I’d say it’s a good way to boost your app views and downloads, at least it worked for me 
And it’s faster than forums and reviews!
Tappx - Cross-promotion platform for Apps - The App Marketing Platform, Tappx
Promote app on forums is a good free app marketing method, but not so efficient, because most people discover new apps by browsing or searching on the app stores. Then the best way to promote app is increasing app visibility on the app stores. Following the following FREE methods to increase your app visibility:
Boost App Ranking on The App Stores
When searching or browsing on the app stores, most people just browsing the top 10 apps. You can target keywords with low competition and lower search volumes firstly, which are much easier to rank for, you can boost your app ranking with the following methods:
[ul]
[li]Use Keywords in the Name and Description - Data shows that adding keywords in title will increase your app ranking by 10.3%. [/li][li]Update your App Regularly. Updating app can increase app ranking temporarily.[/li][li]Gather Traffic and Downloads[/li][li]Increase Engagement and Retention[/li][li]Regularly Update and Upgrade[/li][li]Reduce App Size[/li][/ul]
Increase keywords coverage
The more keywords your app covers, you more chances it will get to be noticed and installed. Follow the following methods to increase keywords coverage:
[ul]
[li]Avoid Repeating Keywords[/li][li]Use digits not words[/li][li]Use Shorter Keywords[/li][li]Wipe off connectors and stop words[/li][/ul]
Besides increasing app visibility on the app stores, you should get positive app reviews to incease the download rates, because 88% users trust reviews as much as personal recommendations, and 95% users will take the app reviews as one of the most important reference factors to judge the app.
Hope these methods are useful to you.