From HBS Working Knowledge, another article on E-mail Overload:

Before e-mail, senders shouldered the burden of mail. Writing, stamping, and mailing a letter was a lot of work. Plus, each new addressee meant more postage, so we thought hard about whom to send things to. (Is it worth spending thirty-two cents for Loren to read this letter? Nah….)

E-mail bludgeoned that system in no time. With free sending to an infinite number of people now a reality, every little thought and impulse becomes instant communication. Our most pathetic meanderings become deep thoughts that we happily blast to six dozen colleagues who surely can't wait. On the receiving end, we collect these gems of wisdom from the dozens around us. The result: Inbox overload.

Among the tips:

• Use subject lines that tell people something about the email
• Refer back to the original question when you provide an answer
• Make what you're asking clear and easy to pick out of the text
• Get out of the 'always-on' mode of checking email
• Delay your response (this slows the conversation to a manageable level when the conversation is important, but not urgent)