Once your new guitar strings are on, tune up. Then go to each string and pull it away from the guitar body by about 1/2 inch to an inch. Don't pull too hard or too far. You just want to help seat the the strings wraps around the tuning post. Play the guitar for 5 minutes and retune. Play for another 5 minutes and tune again. After three times your guitar strings should be in a pretty stable tune.