We've had very good luck with making our system an invite only beta. The main site is setup to very clearly explain our product, and it has a form to fill out

We've had very good luck with making our system an invite only beta. The main site is setup to very clearly explain our product, and it has a form to fill out requesting an invite to the beta. Then we go through the requests and slowly grant access to the early stage product. We can both filter users (to make sure they're our target group) as well as convey to them that this is super early stage of development. Plus, there is definitely a psychological influence of people getting an invite to a beta; we've had a much higher percentage of invited users create beta accounts compared to just allowing anyone to create an account.