I'm sure you've heard the advice, from many time management experts, that you should only touch something once.  That when it comes to you, you should either do it, delegate it, defer it or delete it.  This makes complete logical sense but is much harder than it sounds, especially when you think about the reams of email received each day.  Chet Holmes, in his book "The Ultimate Sales Machine" recommends that organizations implement policies that insist on very descriptive subject lines for all emails.  This way you can make the decision, with one glance at the subject line, on whether you can deal with the email immediately or not.  If you do end up opening it, despite the descriptive subject line, but can't do, delegate or defer it now, then he recommends that you move it to an appropriate folder and add the item to your physical to-do list.  

Even if your company doesn't implement this policy, you can help your co-workers and clients by using very descriptive subject lines yourself.  And hopefully they will see the advantage and begin to do the same.