It is becoming more and more important to get your market involved from day 1 of development (as opposed to day 1 of launch).
How do you engage customers throughout the product development cycle, and keep them engaged?
As we are entering beta, I feel like we've missed out a bit on the feedback we could have had already, had we engaged the market prior. So really I'm looking for methods of engagement. Is a blog or newsletter the best form of engagement? How about social media?
I don't want to have too many vehicles of engagement, just one - maybe two. So narrowing down to the best one or two is my goal. I'm looking to draw on the expertise of the community.
Since you clarified to state mainly BETA testing... my suggestions would be to try to give as much information as possible. Let them see what you are working on / fixing / changing per feedback. I think that this helps them know that their time is actually valuable.
If your site has badges, subscription or something else for users, offer them something unique for helping you out. It is amazing to see how much more users are willing to endure to be able to show that they were "There First".
I think for BETA you want to keep it off of "public" sites if you are seeking honest feedback and do not want any potential issues to harm your reputation (why you are probably in BETA). However the best thing I have found is to give your testers a feeling of community, it will create a fierce loyalty base and start to get word of mouth spreading. Forums seem to be the best way to do this at this time, let them post/comment/share, and have your team be active as well.
Books: Two books that I know of that give specific step-by-step, actionable road maps for how to do this best are The Entrepreneur's Guide to Customer Development - www.custdev.com - by Brant Cooper and Patrick Vlaskovits and Ash Maurya's Running Lean - www.runningleanhq.com. For example in the custdev book you first validate your core hypotheses regarding your product. This consists of testing for Customer - Problem - Solution. Then building a MVP. Then achieve Product Market fit and finally, produce a development and marketing roadmap for scaling. There are feedback loops at each step and the book walks you through that process. Running Lean is in a similar vein. Just a couple of quotes to give you a flavor of what it is about: "Running Lean is about testing a vision by measuring how customers behave." "Running Lean tackles both product and market validation in parallel using short iterations." Both of these books follow the lean startup philosophy. Also, read Steve Blank's blog. www.steveblank.com See his startup tools tab.
Tools: Here are a three tools I've learned about that you should consider for user feedback - survey.io by Kissmetrics (recommended by Sean Ellis) and Usermood - www.usermood.com - by Dave Churchville. Another type of tool you could use would be FeatureSet - www.featureset.com - which is Social Software for Product development. Kind of like a project management tool that includes your customers as part of the team. I'm sure there are lots of other tools so it's worth searching around a bit to find one that suits your situation.