The frequency will help increase your awareness of how you’re sitting, and if you do this a few days in a row, hopefully you’ll have to do less to correct your posture each time.
In addition to the posture check, when your timer goes off, take a deep breath or two. This will help your muscles relax so that when you adjust your posture, your muscles aren’t so tight.
A deep breath can also help you focus on the task at hand. You may also find yourself yawning less, since a yawn is essentially the body involuntarily requesting more oxygen.