1. Don't Over-apply Your Antiperspirant/Deoderant

You know what's causing that yellowing stain: probably aluminum. Switching to a deodorant without aluminum can help with permanent staining.

2. Air Dry Your Shirts 

Heat can make pit stains permanent on a shirt. Air dry and keep especially sweaty shirts out of the hamper to get rid of pit stains easier. 

3. Keep your pores clean

Sweat is natural, but dirty pores don't need to be. Consider using Vanish or another acne spray to keep your pores clean, even as you sweat.