If it is something that requires action--a bill, sending a card, returning something--it goes in a pile to be acted upon. Set a time on your calendar to attend to these! Is there time right after you are done sorting? That is the best option.
Maybe it is something that you need to keep button action is required--car insurance policy, statements from your broker--so your second pile is for filing. Set another time on your calendar to file these if it is a large pile, or handle the sheet only once by filing it now. I like to do this once a day so that the pile never gets too large.
Ditch is the other option. This is something you don't need or want anymore--=the thank you note for a gift you sent your aunt, the hard copy of the webpage for the blouse you thought you wanted to buy and decided against, a solicitation for money to a group you don't wish to support or what many simply just call JUNK MAIL.
Sometimes, you aren't sure if you should keep it. Ask yourself : Is it something that can be replaced if I need it? Maybe by going online? Or scan it into your computer for later reference if the need arises. Good options if you aren't sure.
Get rid of messy piles of paper on your desk and counters. You'll feel much better when you do!