The easiest way to save money? Start a budget, and track everything you spend. Seeing how much you spend without even realizing it is an eye-opener.
Except for the rare circumstance of the extremely disciplined individual, I'm a firm believer that whatever money we have, we will find a way to spend. So invest or save (move) as much as you can, automatically, before you have a chance to see it. Increase your 401k contributions. Set up automatic weekly transfers to a higher-interest savings account. Contribute more to your HSA or 529 plan. Whatever floats your boat. Just do whatever you can to not have the money immediately available in your checking account that is just 1 swipe away, thus reducing the starting amount for your budget. It's amazing how simply seeing a lower account balance subconsciously (or even consciously) makes you spend less. And you're saving money away in the process. It's a double whammy.
"Live within your means" is the obvious one. Don't spend money you don't have. But in the event you do have (and use) a credit card, set it up to automatically pay in full every month. Credit card debt is the beginning of the end.