Many different options:

• Train the Engineering Skill

This is the most effective option. You gain 5% more power per level. Every character starts with this skill at level 3.

• Use a module that increases your power grid by a fixed amount

The Micro Auxiliary Power Core I module and its relatives increase your power grid by a fixed amount. They are only useful on frigate and destroyer sized ships as the amount is quite low. From 10 to 12 MW of power grid. This module requires the Energy Management skill to be trained.

• Use a module that increases your power grid by a percentage

Power Diagnostic Systems give you a 5% increase in power grid and a boost to many other stats (capacitor recharge, shield recharge, etc.) in exchange for a low slot.

Reactor Control Units give you a larger 10% to 15% increase in power grid. They provide no other bonuses.

Both these modules require the Energy Grid Upgrades skill to be trained.

• Use an Anciliary Current Router rig

This rig gives you a 10% boost in power grid. It requires the Jury Rigging skill to be trained.

• Use a power grid boost implant

The Hardwiring - Inherent Implants 'Squire' PG2, PG4 and PG8 implants can be bought from Loyalty Point stores or from the market and grant 1%, 3% and 5% increase in power grid. They require the Cybernetics skill.

• Train the Advanced Weapon Upgrade skill

This skill (which required Weapon Upgrades V) will reduce power-grid used by your weapons by 2% per level. Thus effectively "gaining" power grid to use. Many fits on websites such as battleclinic or scrapheap-challenges expect this skill to be trained, sometimes even to level V to work. It's usual to specify whether the skill is needed and to what level.

Recommendations

In general, it is a good idea to train Engineering V as soon as possible as it is pretty quick to train and universally useful.

The Advanced Weapon Upgrades skill is very useful in fitting high-end ships, especially with T2 weapons (which use about 10% more power grid) and T2 ships.

Fitting modules and rigs should be used in moderation as they use a slot that could be otherwise used for more armor, damage, speed or similar upgrades. Some ships however are almost universally used with such modules. A good example are "fleet" sniper battleships, the massive fitting requirements of the largest T2 turrets often make a Reactor Control Unit II mandatory. Many frigates also have popular high performance fit that make use of auxiliary power cores to fit microwarpdrives, high-calibre guns, 400mm armor plates or medium shield extenders.