It is good to know your fuel consumption and how to maximize it.
Bottom paint is a huge one. Mooncratered hulls can really suck the fuel down.
Weight in another, especially on small boats. Lighter is not always better on mid size and larger boats. My boat runs faster with full tanks than near empty tanks. The hull like a lot of weight midships. My tanks are close to full most of the time.
I wouldn't recommend going out and buying a flowscan though. Everyone knows what rpm maximizes fuel flow from any chart available on the web. It will take a long time to save enough gas to pay the thing off when you know what rpms are best without the flowscan. Flowscan is better suited for twins that need to synchronize and burn a ton of fuel. Not really needed for a 4 banger I/O, won't pay you back anytime soon, or later either.
The hydrofoil fins on small boats will also save a ton of gas if you jump on plane often skiing or running drifts for fluke fishing. One of the cheapest upgrades that will pay you back many times over. 50 bucks for immediate payback.
Stripping and setting up the bottom with the right low friction paint is a lot more work but starts paying back immediately.
Another side note is props. There are a lot of boats out there that are overpropped/overgeared and lugging. Make sure your boat jumps right into the max rpm rated range window without making you wait and without porpoising, etc. The boat should be peppy and jump right up onto plane, full tanks or not, without playing with trim tabs for 5 minutes. Trim tabs are not for planing except in the case of a boat that could be set up a lot better, they are using one at a time for correcting a slight list from a quartering sea so you can maintain a course more comfortably in big waves without having to zigzag and waste more fuel than necessary.
Keep the canvas rolled up. A parachute. Parachutes add drag, use more fuel.
That's all I can think of right now.