Like pdelvo already said creating a great app is the first mandatory step to reach an audience.
As a single dev I admit it is really hard to promote your game. Here are the few things I tried so far with more or less success:
Adduplex: A good tool but it can't do miracles, I earned 1 000 000 ads impressions thanks to devcenterbenefits program, you can also get some credits from dvlup, I wanted to use all of them in a week, but the network can't handle this and right now I'm printing around 30 000 ads a day.
Twitter: Using the proper hashtag and tweeting to the right persons can be an effective way to reach more people. I had the luck to be noticed by a Brazilian Windows Phone dedicated website just by tweeting my trailer with #wp8 or #lumia hashtags.
Facebook advertising: I tried to spend some money in facebook advertising, at first it seemed nice, you can precisely target your facebook page with direct link to the game to people who like windows phone in the US for example, and it worked I got many likes this way, before realizing that people behind the likes where from click farms and that this was just a scam.
Beta: I started a public beta few days before releasing the game, but failed to gather enough testers to make it worth it, so I thought it would be better to focus my effort on the actual game release rather than on a beta.