One tip? Take your time with it (i.e. hold the stretched position for
at least 30 seconds), and do it as frequently as possible throughout the day. Think about trying to stretch a leather belt -- it's not going to produce any tangible results if you just tug on it once a day for a few seconds.
OK can't help it - tip two (much more important):
remember that everyone's different; don't try to be "as flexible as that person", or to be as flexible as you think you "should be". The key IMHO isn't achieving some ultimate flexibility goals, but just keeping things (in your body) moving around frequently enough, and with enough range. IOW, don't let your "I'm not very flexible" thought bother you. "I'm not moving enough" is a much better formula for concern.