Use it while traveling. This will alllow you to remain alive long enough to get past a mob without needing to defeat it.
Rejuv is a cheap, effective HoT. Use it when you or your teammates are moderately damaged or are not in immediate danger of being hit. At lvl 64 and beyond, [Lifebloom] can be substituted for this purpose.
Use it on the tank(s) before a pull (before they enter combat) so that its ticks will heal the damage of the first few hits done to him without putting you into combat, reducing your threat and increasing the tank's aggro (since the mob thinks he's healing himself).
To increase Healing per Second, use [Swiftmend] after the 3rd tick, and recast Rejuvenation right away.