Im a Beta Tester and have Beta Tested iOS6 and 7, Procreate, 1Password 4 for Mac, Pixel Press (still in the early Alpha stage) and ill be testing The Long Dark for Mac sometime next year, as well as the next upgrade of TextExpander by Smiles Software.
Its a matter of really getting into Social Networking, on FB and Twitter and following your favourite App, and then you get to know the Dev's and you follow them, and you just keep pushing, and pushing and pushing.
Thats what i did/do and thats really how im on the list for the next upgraded TextExpander. . . . You would be surprised at the amount of FREE Mac and iOS Apps i have got over the last 2 yrs. All in all, i have saved $300+
Emails back and forth and Twitter have been my best avenues as FB its out in the Open and i like to be behind closed doors.
Another way is to look at places like Kickstarter, Indiegogo and SellanApp . . . .
The first 2 are crowd funding places and if you back a iOS App or Mac App then they usually have a Beta Testing Pledge and you get in the door that way. Thats how i got Pixel Press and The Long Dark, and because of this, i have become friends with the Devs and ill be on future Beta Testing for other Apps now.
SellanApp is a little different but if you have a App idea, then you can put $ into that App and again, become friends with the Devs.
There is now easy way in, but just keep trying and as i said, if you have a favourite App, follow that App, and the Company on twitter and let them know your there to test if need be. If they are a successful company, then they will be making more Apps, which is more opportunities
Good Luck