Articles About Beta Testing

5 Ways To Make A Tester’s Life Less Miserable (DevMag)
Ron Carmel’s Rules For Playtesting (2DBoy)
Tutorial: Playtesting (Beta Testing) Games (Studio Eres)
Valve’s Approach To Playtesting – The Application Of Empiricism [PDF] (Valve Software)
10 Tips for Better Playtests (GameDev.net)

7 Reasons Why You Should Record Playtests (Black Golem)
Solo Testing (Game Design Concepts)
Playing With Designers (Game Design Concepts)
Playing with Non-Designers (Game Design Concepts)
Blindtesting (Game Design Concepts)

How Many Players Should You Playtest With? (Gamasutra)
Making Your Game By Breaking Your Game (The Game Prodigy / 6 min video)
Discussion About Beta Testing (TIGSource Forums)
Power To The People Test (develop)
Successful Playtesting In Swords & Soldiers (Gamasutra)

How to Playtest (Throw The Looking Glass)
Level Design Primer: Notes on Playtesting (gausswerks)
Interpreting Player Feedback (Derek Yu)
How To Beta Test Your Game (Page 4 in the free eBook “How To Sell Your Game”)
Game Testing (Wikipedia)

Find Beta Testers

Beta Testers: Getting Them. Keeping Them. (Jeff Vogel)
Finding Testers (gamedev.stackexchange)
Paul Eres Does Beta Testing For A Fee
Let’s Players on YouTube (some of them might be interested to test and provide feedback while recording a video of your the game)

Furthermore

A live video streaming service might also be useful for instant feedback.