There's always a risk when you take on a beta tester, you never really know if they're in to get early access to the game and then sod off afterwards. We've been pretty lucky in the time I've been with the team (Sept 2007) in that I think we've only had two like that!
We try and vet people who apply. First we get them to fill out a questionnaire and then set them a bug hunting test on TIC. While they are doing that we check them out on their posts, to get a flavour of the type of person they are, which is why we like to go with guys with a good track record on TWC.
Joining the Beta testing team is also a potential way of becoming a full team member. Many of the guys started that way, myself included. Anyone who shows initiative, a willingness and ability to learn, a passion for the game can make it in. When I joined I only thought of being a tester as I wanted to help out with the first RTW mod I ever tried. Now I seem to have a finger in most of the pies going - lol.
RS is probably the biggest RTW mod out there at the moment, so I guess you're attracting a lot of the kiddy winkies, very much like Stainless Steel does. This means you are probably getting more than your fair share of time-wasters, which is a pain in the arse when you're trying to get a job done!
Sometimes though you hit pure gold! I know Brusilov does some work for you, and I'll bet he's as highly regarded with your people as he is with ours.
Another thing we do is get new people to post a statement of intent, which includes a confidentiality clause. Of course we can't enforce it but it at least makes the individual think about it.