This free guitar lesson by Peter Vogl will teach you several drills to increase your ability to bend notes. It's important to practice your bends so that you are accurately hitting the correct pitch when you play half and whole step bends.
Watch the lesson: Quicktime
We're going to play a G Major Scale starting at the 2nd string @ 8th fret.

Now we'll incorporate some bends into the G Scale. We'll start by playing an ascending scale.

And now a descending scale

Practice these bends along with the video until you are comfortable with them.
Complex Drills by Peter Vogl is an instructional video that covers more technique exercises. Building on what was taught in the Warm Up and Drills Download, these drills will help you build more hand strength, dexterity, develop good hand position, economy of motion, and of course speed. Practice is the key to becoming a better guitarist, and these exercises should become an integral part of your practice routine. $4.99