Most of the time I just leave it as is. I let the staff know I'm leaving but won't say anything to members its only for a few days. Now If I'm going to be gone for a longer period of time I'll assign an Assistant Administrator while I'm away. So it's worked out great and I've been doing it this way since 2005.
I have a Mod and then another Admin (more a super moderator really) so if I have to go away, they are pretty prepared for it all. If they need any help or my opinion though they will text me and I will help them if need be, but this hasn't occurred. ATM though I'm all on my lonesom because they are both taking a break for personal reasons, so I have no choice but to hang around, luckily my members are all great so it's relatively easy going.
I have two other admins that know how I generally run the forum, and they act according to my specifications. They know how I would react in certain situations, and make judgments based on that, rather than just acting on their own. And I've seen them make such actions before, and they're decisions were directly inline with what I would've done. They also have my Facebook, MSN, Email, Skype, and Mobile Phone number just in case. I check all of those on a regular basis on my mobile.
I actually tell my staff members that I'm taking a leave and that they should remain focused on building the community. There's no "I" in team.
I'd let another admin run my forum while I'm away, but I don't ever see myself being away for more than a couple days. Even if I'm away I'll still access my forum on my phone.
I just like to build a strong enough team that any one person (even me) being absent wouldn't bring the site any harm. Any forum that revolves so much around a single person that their absence would be devastating is more of a cult of personality than a community.