To minimize maintenance I wanted to run Jenkins as a different user that I already use for other services, as to avoid permission control hell and ssh-config hell as well.
So here's what I did:
Let's say the user you want is called "johndoe" and it has an userid=2000 and groupid=2004 in your box, do this:
1. Edit /etc/password
2. Change permissions so they reflect the new ID
chown jenkins:jenkins -Rv /var/cache/jenkins/
chown -Rv jenkins:jenkins /var/lib/enkins/
I wrote this last minute and after altering things for the second time, (things got a bit reverted after upgrading Jenkins in my Archlinux home server); so let me know if this doesn't work for you and I'll check.