ok dokie... i havent been a beta tester for anything but from what i have read her eare a few sergestiions
1- learn a skill that will be useful to the company who wants beta testers, coding and moddeling expertese are 2 that come to mind, you must then be able to prove that you have these skills and be able to show the designers of the object you are beta testing the problems with the game/software and possible changes and fixes that could be utilised to get that game/software back on track.
2- you must have an eye for detail. Being able to beat a game or use a software is not a useful trait, you need enough skill to be able to actually get somewhere in the game, but if you ignoor all of the minor glitches or even dont notice them you wont be much use to the company that has let you test there software.
---- for example, it is possible on quite a few games to walk through the corners and edges of the game world because they are not properly connected, if you have steamed through the game you will probably not have noticed this, but if you take your time and test every aspect of the game the chances are you will find something.
3- patience is a virtue, remember while you are testing this game/software that it is still in development and, being a beta is not meant for public use, the chances are that it will crash allot, glitch allot, ect... if you have trouble holding in your anger at a simple crash on a normal game avoid beta testing alltogether, or you could end up with quite a hefty computer repair bill each month.
.. ho hum.. thats all that comes to mind at the mo, other than keep looking for places who want beta testers and signing up to them, keep active in areas and make yourself known.. and enjoy it..
oh yeah 1 more thing... if you dont like writing then it is probably not advisable to try for beta testing either, some of the bigger companies tend to ask for reports each week/month as a sort of checkup
good luck

and hope that helped