This month I'll be flying twice within the United States. I am a US citizen with a valid drivers license, flying within US boundaries. Is this sufficient to fly? I haven't held a valid passport for over 12 years and am uncertain of the flying restrictions in this day and age. Will I be OK to fly without my Passport?
You don't need a passport to fly within the US. As long as you have your driver's license, that is enough to get you through TSA check points. It's when you travel outside the US that you will need your passport.
Here's a list of acceptable IDs from the official TSA website: http://www.tsa.gov/traveler-information/acceptable-ids
The main changes since the last time you flew involve all those other places that used to not require a passport. e.g. Whereas previously it was possible to visit Canada or Mexico with just a driver's license, you now need a passport.
Within the US, AtlasRN is 100% correct. Your driver's license will be sufficient. What you will find different these days is the amount of security checks, scans and restrictions. You will be wise to wear shoes that are easy to remove, read up on the restrictions for carrying liquids in your carry-on bags, carry electronics (especially laptops) in a bag that makes them easy to remove, and allow extra time for getting through security.
Beginning October 10, 2018 A driver's license from a State that does not meet "Real ID" requirements is not sufficient to fly between destinations in the US.
1.Check your state’s status: You can check if your state is REAL ID compliant at dhs.gov/real-id-enforcement-brief. Beginning January 22, 2018, travelers from non-compliant states will not be able to use their driver’s licenses or state IDs at TSA checkpoints, unless the state received an extension to comply. Source (PDF)
Currently (November 2017) only about half of US States have compliant licenses. There are a few other acceptable options but for most people a Passport is the most likely choice, and the Passport card being the most cost effective for domestic travel only.
Beginning October 1, 2020, even if your State is compliant, your driver's license needs to be compliant (you might need to renew it).
The list of compliant States is can be viewed at https://www.dhs.gov/real-id#
Currently all States and Territories are either compliant or have an extension, except American Samoa. Those extensions expire October 10, 2018, at which point your driver's license may not be sufficient