Improving posture is not easy and doesn't happen overnight. After receiving chiropractic treatments, you'll already notice that you can maintain a more upright posture for a longer period.
The tip is always self-observation. Start noticing when you cross one leg over the other (and especially if you always cross the SAME leg), which can lead to misalignments in your pelvis. Also, observe if you have a tendency to raise your shoulders (or keep them elevated), as this creates a lot of tension and fatigue by the end of the day. When standing, try to evenly distribute your weight between both legs instead of shifting the support from one leg to the other. You will notice that with these changes, you'll feel lighter and more vital. The important thing is that these small practices don't cause you tension or concern when doing them, so start by simply observing your body.