Some races, you just have to volunteer. Others, you have to be a member of the group (such as the local running club) that's agreed to provide pacers. Of course, you will be asked to show some evidence that you can actually maintain the pace you're volunteering for.

Personally, I would never volunteer to pace because I run negative splits. But I stand in awe of the people who do pace. For a race that I help administer, our pacers are amazing - this past year, 11 of 12 finished within 60 seconds of the advertised total time for a half marathon.