I am currently going through this too, the approach that we are taking is contacting users who have previously responded to feedback forms.
We have in the past asked users to provide feedback on our app through support forms and surveys, we are now emailing these users saying something along the lines of:
Would you like the opportunity to give 'us' regular feedback?
You could join our usability testing panel and you will be rewarded for sharing your views on new features or designs
They then have a link to a demographic survey so we can add them to our list.