There several different types of trigger lock show below.

These are very inexpensive less than $15 dollars and for the package of six I have found them for under $30 dollars.

The most appealing features are they are widely available from your local hardware store to any Wal-Mart store. They may prevent discharging the firearm but it cannot prevent someone to loading the gun. The first rule for gun safety is never leave a gun loaded and always act as if it is loaded, never pointing at anyone. Caution should be taken never load with the trigger lock in place  under certain circumstances the gun could fire a trigger lock cannot be used on a lever-action  rifle or shotgun, keys and combinations must be kept away from children and other unauthorized persons.

Cable locks are another way to make your firearm safer it is a gun lock system that disables the firearm from being fired.                   They can be used on wide variety of firearm from bolt action to semiautomatic. The cable gum lock is very available and be found in the same places as the trigger locks. You can use this style of locking system to lock the gun to a solid object. Caution should be taken to insure to install according to the direction, not around the trigger and keys and combinations must be kept away from all unauthorized persons and the cable could be cut with a hacksaw or with a set of cable cutter.

Disassembling the firearm to create a condition with that firearm to a state that the gun cannot fire or be discharged is another safe way to insure peace of mind on keeping unauthorized persons from firing the firearm. On a bolt-action rifle the bolt can easily remove and stored somewhere secure. On semiautomatic the trigger assembly can be remove with simple gun punch and stored in a secure place. On a revolver the cylinder can be remover and place in safe keeping somewhere secure. The goal should be to make firearms safer.