If you want your app to succeed one thing you need is reviews. More importantly you need good reviews!
Stats show that if an app has a rating of 4 stars or above it will do significantly better than an app with less. The higher the number of reviews, the more believable that rating is.
One issue you may have though is how to get reviews. Sadly most users don't rate apps unless they really like it or hate it. The Windows Phone OS also does nothing to prompt people to rate apps, so if you want to build up reviews then your going to need to do the prompting yourself.
The code to direct someone to the review page in the store for your app is quite simple. But even better is the fact Nokia have produced sample code that does the whole process!
Once you've integrated the code on the 5th launch of your app the user will see a prompt asking them to rate your app. If they choose not to then they will be prompted to send you feedback via email.
If they didn't rate your app then on the 10th launch they will be prompted again.
Now getting good reviews is slightly harder, for that your actually going to have to build a good app that people like.
You can download the Rate My App code from Nokia here.