Before each trip, you should always check the following:

Inspect all lights to insure they are functioning properly
check your tire pressure and visually inspect all tires for wear
Inspect the coupler or actuator for damage
If your trailer has brakes, be sure to test them before you get out on the road.
Check condition and tightness of bolts and fasteners.

When your boat is off the trailer, be sure to do the following:

Inspect the bunks or rollers to ensure they are in good condition and working properly.
Inspect the winch strap and safety chains to make sure they are in good condition and not showing signs of wear.

After each trip:
It is always a good idea to rinse your trailer thoroughly with fresh water after salt water use.

Always refer to your owners manual for a complete list of service and maintenance requirements.