Using constant and immutable variables.

For variables, that won’t be ever changed you can use constant or immutable keyword. Constant variables must be fixed at compile-time, immutable variables can be assigned at construction time. Using these keywords not only saves gas (because such variables don’t occupy storage slot), but also improves readability of code.

In Solidity state variables are stored in 256-bit (32-bytes) storage slots, but multiple contiguous variables can be packed in one storage slot if their overall data is less than 32 bytes. Here is example: in first case every variable takes one storage slot, in second case we save 2 storage slot, because variables b, d, f are packed and saved in one storage slot (128 + 64 + 64 = 256).

 Incrementing and Decrementing by one.

There are several ways for making incrementing or decrementing by one. Most effective method - prefix increment/decrement.

 Use unchecked block.

Since Solidity 0.8.0 version compiler automatically performs overflow/underflow checks, that requires additional gas cost. If you are sure, that overflow/underflow is impossible you can use unchecked block to skip checks and save gas.

You can use Solidity compiler optimizer for your code and setting optimizer “runs”. Increasing “runs” decreases calling functions gas cost, but increases cost of deployment smart contract. So test several values of optimizer “runs” for getting better balanced optimization.

Use revert instead of require.

Instead of using require for checking statements, use revert with your custom errors to fail transaction for saving gas.

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