If your app is good and has some cool features sites like WPCentral, “WMPoweruser” , WinSource, Windows Phone daily can cover it. You can get your downloads to increase. I had mine peak to around 4000 in 2 days when it was featured on WPCentral.
Also if your app is featured in the store even better. However you cannot control that.
Post in places like WPCentral forums, Reddit and BTW there are massive groups on facebook some with 80,000 members so you can promote there to with the admins permission.
Next you can also use DVLUP, Twitter for promotion. Create Facebook page and twitter profile for your app and try to maintain them.
You can also try to register for apps like MyAppFree, 1800pocketpc (don’t know if it is still active) and more.
And finally most important of all, keep updating and working on your app.