The most important boat trailer maintenance task is to lubricate the wheel bearings. Otherwise, trapped water will wreak havoc over the winter months. Begin by jacking-up the trailer, pull the tire and wheel. Remove cotter pin and axle nut, then slide off the hub. Reach inside the hub and remove both bearing cages. Soak them in solvent to remove the dirty grease. Do not make the fatal mistake of drying the bears with compressed air because the air pressure will spin the rollers and burn them out. Also, clean the bearing races located inside the hub. Pack the bearings with marine grade grease and re-assemble.
Walk around the frame, looking for rust and cracks. Sand any rust down to bare metal, then prime. Primer is best applied in a thin coat. Let it dry completely in order for its volatile chemicals to out-gas. Rebuild the finish with several coats of paint. Apply in a smooth swipe, don't swish the can back and forth or the excess paint will accumulate and sag. Take your time for a professional looking finish.
Cracks and breaks in the frame should be welded as soon as possible before they worsen. Examine the leaf springs for broken leaves, but do not attempt to repair one yourself Instead, seek professional counsel. Inspect the shackle bushing by removing the shackle bushing plate. This is the assembly that bolts the spring to the frame. The rubber bushing that surrounds each bolt should tightly grasp its bolt without slop. Worn bushings can be pulled out with a pair of pliers and a new one tapped back into place with a rawhide mallet.
Inflate the tires to full pressure. Sink a penny into the tread. The coin should sink into the groove as far as Lincoln's beard. Any less and the tread is excessively worn. Inspect the sidewall for cuts and breaks.
Make sure all of the bunks or rollers are adjusted to support the weight of the boat. If any are out of line, back the trailer into the water just enough to make the hull buoyant. Adjust the height. In colder climates with bone-chilling water temperatures you may prefer waiting until spring or summer.
With the electrical connector plugged into the tow vehicle, check all of the running lights. Replace burned out bulbs as well as any cracked or broken lenses. Either a healthy dollop of vaseline or grease smeared on the lens gaskets helps keep out water.