I don't do anything with items in my "Sent" email folder because I don't have a need. If I send an email with a request to someone that I need to track it goes in my Waiting For list. I've rarely had a need to dredge up sent emails, and when I have I've been able to use a combination of my email client's search and sorting functionality to find what I need relatively quickly (at work I've always used Outlook).
But... that's me. Maybe your situation is different.
Why are you concerned about emails in your "sent" folder? It sounds like it's just a vague fear that you ought to be doing something with them; if I'm right, my suggestion is to leave them alone and not create unnecessary work for yourself.
On the other hand, you may have a specific need that wasn't clear from your original post. If that's the case, it would be helpful if you could further explain the challenges you're experiencing (such as why you need to look up sent emails, what email client you're using, and why you're not able to find them efficiently now).