You might see the following error when trying to deploy either a test or final package build of your game to your device:
This means that the WP8 app deployment tool won't send the .xap to your phone because you already have too many developer apps installed (copies of your other projects). Wingows Phone 8 can be very restrictive in this regard - often allowing only a single developer app to be present at a time . Please delete other installs of your project(s) via its icon in the Games app. You don't have to restart GM for the change to take effect and your .xap should now deploy fine again. If it still does not, please restart your phone and your PC before trying again.
The number you're allowed depends on how and when you got the MS Account you used to developer unlock the phone - if you're using a free license you will be locked to at most 2 developer app "side-loads", but if you have a full paid license you should have a maximum of 10 installs.