1) Insist that people sending me e-mails use *descriptive* subject lines; then use the subject lines to "triage" my e-mail as to what needs to be opened immediately; what can be put off; and what can possibly be put off forever if necessary.[this is my most critical e-mail management tip; as I get so many e-mails I'd never be able to handle them all no matter what]. A "descriptive" subject line need not always be lengthy, even, just accurate and with enough information to allow the reader to judge the e-mail's likely subject in a general sense.
Example: "House Mark", a two word e-mail title, is VERY descriptive; as is "Hearings"; both would indicate to me an e-mail to be opened immediately; whereas "That Thing On Tuesday", twice as long a title, is completely and totally UNdescriptive and generally useless in judging anything about the e-mail or its importance/subject/urgency.
2) Folders. Folders. Folders. I have a wide range of folders for various topics; I also have an "Ongoing" folder for issues unlikely to be solved any time soon that aren't otherwise specific and that I'll need to access frequently.
3) The "search" function.
4) PDF'ing e-mails relating to tasks I haven't dealt with as of when I leave so I don't forget they exist during all of the *next* day's crises.