Keep a questionnaire short and easy to fill out. Ask a few specific questions and leave some space for comments. If you can offer a small reward for filling it out, it helps. People will respond well to positive, upbeat and confident emails that suggest their feedback will be utilized. I like contributing to small projects and if you can let them know their feedback affected the system.
Numbers are useful (such as a likert scale), but if you have the resources to wade through some anecdotal/qualitative data - get it.
I like google forms.