What I do to stretch my strings is them up... and tune them so the strings aren't flopping around... they don't have to be tuned to standard tuning or anything... just tighten them up a little. Then lay the guitar on your lap so the strings are facing you....then hook a finger underneath one of the strings... I usually start with the low E string.... Next pull the string toward you as much as you can. You can pull pretty hard... the string may seem like it's going to break but it shouldn't. You should be able to pull on the string until the guitar lifts off your lap... Hold it there for a few seconds then let it down. With the same string in hand... Stretch it and pull it toward the treble strings and back across the guitar... do that back and forth a couple times... Then move to the next string and do the same thing. Obviously... wound strings will need more work than gut strings. I usually work the wound strings twice and the gut strings once... Once you're done stretching tune the guitar up and stretch them again... this time with less force... Then tune them up again... this should have elleviated most of the elastic energy in the strings.... allowing them to break in a lot faster and stay in tune a lot better.