String bends and vibrato are among a guitarist's most expressive techniques. Testing the accuracy of your bends and vibrato is a good way to judge how developed you are as a guitarist and a musician. Accurate string bends and vibrato don't come from your hands but from your ears. They can't be practiced mechanically like alternate picking and sweeping.
It takes a more careful approach to develop your ear to hear pitch. I've heard many players who can play extremely fast, accurate scales and licks but can't execute a simple string bend and stay in tune.
Here's a quote from Yngwie Malmsteen: "A lot of people don't realize that guitar playing is very much like singing or playing any of the glissando-type instruments. You have to do it in tune."
Here is a simple lick inspired by Gary Moore's rendition of Roy Buchanan's "The Messiah Will Come Again." The lick is played entirely on the 15th fret of the B string.
For the first phrase, play the root and then raise the pitch of the note by a half step for each bend (D-D#-E-F), then come back down to a full step (E). For the second phrase, alternate between a full-step and half-step bend (E-D#-E-D#) and finish by coming down to the root (D). Here is the audio of me playing the lick:
You should add some vibrato to last note of each phrase and then record yourself playing the lick and listen to see how well you stayed in tune. If you have trouble hearing pitch, then record record a guide track playing the target notes fretted (D-D#-E-F-E-E) (E-D#-E-D#-D-D). Your goal is to play the lick perfectly in tune by ear.
After mastering the lick on the B string, try playing it on the high E string (10th fret) and on the G string (19th fret). Following this, try the lick in different keys to get a feel for bending stings all over the neck. Try changing the key to B and playing the lick on the 2nd fret of the G. This can be a bit difficult!
If you want to take it a step further, try learning "The Messiah Will Come Again" in its entirety and play along to the entire song. I often play this as a warmup because it really makes you focus on the tuning of your playing. As an added challenge, learn how to play the song by ear without the use of a TAB. It's not too difficult as it's mostly melodic, but there are few interesting string bends that might trick you. Cheers!
Will Wallner is a guitarist from England now living in Los Angeles. He recently signed a solo deal with Polish record label Metal Mind Productions for the release of his debut album, which features influential musicians from hard rock and heavy metal. He also is the lead guitarist for White Wizzard (Earache Records) and in 2012 toured Japan, America and Canada. Follow Will on Facebook and Twitter.