In addition to the excellent points above...
Reduce what you take down, carrying extra crap means you have to push more stuff through the water.
Get as streamlined as possible, don’t let stuff stick out unnecessarily, keep you arms close to your body, etc. This might not make much difference at slow speeds, but if you find yourself fighting a current, it can make a huge difference.
Get some efficient fins. Read the tests and reviews and look for a set of fins that tests out at the top end for speed.
Assuming all else is equal, if one set of fins is faster then they must be more efficient.
Even if you’re not planning on going fast, fast fins require less energy at low speeds then slower fins.
Don’t pay any attention to folks that tell you that fin "A" is better against current that a faster tested fin "B". Fighting a current and going fast are exactly the same thing, it’s all about your speed relative to the water not relative to the bottom.
Speaking of current, if you find yourself working against a current, try to get close (real close) to the bottom or to a wall/reef. Water moves slower next to fixed objects. Plus, if you’re careful, you can grab onto rocks, etc and pull yourself along. Just be careful not to damage anything or grab/brush against anything pointy or toothy.