See if you can enlist the help of a friend, then put the jeans on inside out.
Step one: Stand in front of a mirror, and have your helper 'pinch' ONLY the waistband at the back where it gapes. Using safety pins (less 'owie' potential:) pin the pinch-again, ONLY at the waistband, tapering the pinch from the top edge of the waistband to just below it.
*Important note: The pinch will usually naturally fall into a dart configuration, wider at the top edge tapering down to a point; expect that to happen as right and proper, so you don't over-adjust and end up with an adjustment that doesn't give you room to move around wearing the jeans.
Step two: Now take the jeans off and put them back on right-side out; move around, sit, stand, bend.
Still got the gaping going on? Repeat the first step to make further adjustments, then repeat the second step until you get the right fit.
Make sure to use denim needles, and heavy duty thread when you sew the adjustment into your jeans.
Add your voice to the conversation. Click here to answer this question.