Watch the Bends & Vibrato Exercise 3 online guitar lesson by Jeff McErlain from Beginner Blues-Rock Soloing
A great way to practice bending is to fret the note you want to bend to, then fret the note you want to bend from. Get the sound of both notes into your ears, then bend the lower note to the higher note. Don't try to add any vibrato yet, you just want to work on the pure pitch. Listen very carefully to see if you are in tune, and if you're not sure just fret the note you were bending to and see if you got it. Please be patient with this as it does take quite a bit of work. Once again, you're not only working on the physical aspect of bending strings, but also working on your ears. Lastly, make sure to note how hard you're pushing the string.