There must be hundreds of diffrent strategies for dealing with the constant flood of requests that come in through out the day. My personal favorite for email is Inbox Zero. If you havn't come across it before you should go and check it out, it might not be the one that works for you but you should at least take a look.
The thing that really stuck for me was that your inbox shouldn't be a reminder system, most office suites already have two tools that do that better; your calender and a your task / to do list.
Instead of leaving an email that's otherwise dealt with in your inbox thinking "I must follow that up next week when Jo gets back from leave ... " archive the email and stick an appointment in your calendar for some time after Jo gets back that says "Follow up thing with Jo."