Create an ideal customer profile. Create some questions that will allow to you survey a potential tester to determine if they fit your profile. Design simple landing page with very clear value proposition that speaks to your ideal customer.

Ask for a minimum of information up-front (email), but ask for more info after they commit by submitting the first piece of info. (KISSmetrics does a good job of this on their current website trial signup).

Use the their answers to these profiling questions to put the applicants into buckets. Let in the most ideal bucket first, or split them into groups if they're big enough.

Try and measure engagement the best you can. Measure qualitative and quantitative data. Schedule calls with your beta testers to find out more, especially with the ones who's user behavior seemed to indicate that they didn't get value from your product. Find out why. Make sure they are indeed your ideal customer. Pick up the phone and get to know your customers inside and out. Meet them in person if possible. Incorporate their feedback quickly and get more feedback. Rinse repeat.