Permanent Vote by Mail
Illinois now has a Permanent Roster for voting by mail.
If you're a registered voter in Chicago, you should have received a Vote by Mail application in April from the Chicago Board of Elections. To join the Permanent Roster, you would check the third box "all future elections" and fill in the blank for which party's ballot you would like to receive.
There is also an online form at the Chicago Board of Elections.
If you are a registered voter in a jurisdiction other than Chicago, visit the Illinois Board of Elections website. After answering a couple of questions, they will ask you to select your jurisdiction (county), and from there you will be redirected to the appropriate place.