Even if you've packed your carry-on bag wisely and you're perfectly dressed for your security screening, there are still a few things you can do to get through the checkpoint more quickly and easily, and (hopefully) without holding up your fellow passengers or being selected for a pat-down.
First, have your boarding pass and your ID ready -- and ready doesn't mean still securely stored inside your bag. Have them in your hand. Acceptable forms of ID for U.S. citizens include, among others, a driver's license (or other state-issued ID card), passport or passport card, or military ID -- TSA maintains an online list of acceptable forms of ID.
Unless you're a member of an expedited security screening program, you'll be expected to remove your coat or jacket, your shoes, belt and any jewelry you might be wearing (remember, you can streamline this process even more by leaving jewelry and other accessories at home or packing them). Begin this process before you reach the security agent to avoid holding up your fellow passengers -- they don't want to wait for you and hundreds of other fellow travelers to tie and untie shoes. Additionally, your bag of liquids should be ready to be X-rayed. If you travel with your laptop, consider a checkpoint-friendly laptop bag, which eliminates the need for you to remove your laptop from your carry-on for screening.