I was working with a friend the other day and the question of the best way to widen someone's squat stance came up. If you are planning on powerlifting, especially equipped, there are very legitimate reasons for considering widening the stance. Hitting competition-legal squat depth becomes a concern for many so it must be trained appropriately. There are, of course, many ways to accomplish a deep AND wide squat, but the simplest way would be to do sets of wide stance squats as accessory work after completing your regular squat work sets. Care needs to be taken not to overdo this, as you will be stressing adductors/musculature in a different manner, not to mention the new range of motion you are demanding of the hips and hamstrings.
Alternatively, you might try as accessory work:
*(Ultra)Wide Stance Good Mornings - for me, these are essentially wide-stance RDLs with the barbell placed in a low-bar squat position.
*(Ultra) Wide Stance Manta-Ray Squats or Safety-Squat-Bar Squats - the very high-bar position encourages an upright upper body position. This may or may not be what you ultimately want with your squats, so that should be taken into account.
Outside of the gym, I would suggest the stretches listed in Daily Stretches For Strength & Health. Remember that stretches, just like any new training stimulus should be added slowly and progressively.