In addition to switching pillowcases more often as others suggested, you can try routinely pretreating the ones you just took off (ie, give them a quick spray with shout and toss them in the basket). I do this with heavy sweat items like the collars of my work shirts and it definitely helps.
QXJones t1_jd14l9i wrote
Reply to Sheet colors that hide oil/sweat stains? (That are not yellow) by dcdcdani
In addition to switching pillowcases more often as others suggested, you can try routinely pretreating the ones you just took off (ie, give them a quick spray with shout and toss them in the basket). I do this with heavy sweat items like the collars of my work shirts and it definitely helps.