Q: How do I maintain good posture? Whenever I try to sit up straight I can make it for a few minutes, but then I get tired and slump.
 
A: It sounds like your notion of what constitutes good posture involves muscular holding (tension). Truly good posture is relaxed, not “held”, and therefore easy to sustain.
 
Start out by positioning your pelvis well (Lesson 2 in my course, Chapter 3 in my book) with your “tail” behind you. That will enable your vertebrae to stack well. Well-stacked vertebrae allow you to be upright with hardly any muscle tension.

If your pelvis is poorly positioned, it becomes impossible for your vertebrae to stack well, and it takes muscle tension to keep you upright. When your muscles fatigue and need to relax, you find yourself slumping. If you sit well, you will be able sit upright for long periods with no problem.
 
If you have a backrest on your chair, the easiest way to sit is to “stretchsit”, using the back rest to stretch your back.