Here's a basic example, something I once posted on another forum (since I live in Austin I figure going with SRV is only appropriate):
http://www.youtube.com/watch?v=NU0MF8pwktg
Try count the bars when when he starts singing:
"Well you HEARD about love giving sight to the blind..." starts on the 1st bar (I chord)
"She's my SWEET little thing" starts on the 5th bar (IV chord)
"She's my SWEET little baby" starts on the 9th bar (V chord)
Once you can recognize and count the 12 bar pattern you can hear how the whole song is basically the 12 bar pattern repeated over and over.