It definitely takes time and you can help it along. When you are riding, become conscious of your posture. Suck in your stomach and hold it tight and force your body to rely on your trunk to hold it up; don't let your arms do the work. Hold as long as you can, then relax, then repeat over and over for longer times. If your abs are tight and your hands are relaxed you will know you are in the proper position even if it's only for a couple minutes. Practice and it will improve dramatically. If you move your seat back (and down on the post as needed to match height), just do it as little as you can to help with the above exercises. And mark where you "should" be for future reference to move back towards.