I usually create projects with Eclipse and import them into Android Studio, because I hate the project structure AS gives me. In it´s module settings, I have the possibility
The above answer of pyus13 did not work for me (Android Studio 0.8.1).
Instead use
Run > Run "YourProject"
to build the code and create the unsigned apk. You can find your apk file under
$YOUR_PROJECT/$YOUR_MODULE/build/outputs/apk
UPDATE
Still works with Android Studio 1.5.1 You only get new APKs when you have code changes since you last built your code.