Rule of thumb? Small bag under the seat in front of you (where your feet go) large bag in the over head bin. Most airlines have expanded bin space, so try and place them wheels first (or wheels out and up on NWA) and hold onto your coat/jacket until everyone has found a place for their bags.
• Don't attempt to bring prohibited items such as sharp objects or bottles of liquids.
• Stow your bag with the wheels in first.
• Don't put bags in lengthwise.
• Put your larger item up top, smaller item at your feet.
• Use the bin directly over your head. Using a bin up front when you're sitting farther back is cheating.
• Don't commandeer another passenger's under-seat space.
• Make sure your bag is light enough to lift over your head yourself .
• Remove from your bag beforehand any items you may need during the flight rather than sifting through it in the overhead bin.
• Wait until everyone's bags are stowed, then lay your coat on top.