See ATV Winch Cleaning and Maintenance Guide, Part 1 for steps 1 through 10 on how to clean your ATV winch.
Thanks to Todd Sandberg for the following information and photos on cleaning and maintaining your ATV winch.
Winch Cleaning Instructions (Steps 11-22)
a
Step 11
After cleaning and inspection of gears, repack all gears with grease. One thing to note is that the more you put in there, the more resistance the winch will have in freespool mode, especially for those in colder winter climates, as the grease will get a bit tighter in the cold.

Repack the gears with grease.
Step 12
Reassemble gear assembly; then reinstall the complete assembly into the housing.

Reassemble gears.
Step 13
Reinstall the two rectangular nuts in the housing. Once they are put into position, locate the two metal plates that were removed in step 6, put them into position and secure using the two hex bolts which were removed in step 5. Set this completed assembly aside for later use. Sorry, no pics. I forgot to take them for this step.
Step 14
Locate the motor housing and remove the top two hex bolts and the two connecting rods. Lubricate the two hex bolts with spray lubricant and set those aside with the connecting rods for later use. At this time, also remove the two bottom hex bolts which retain the two metal plates to the housing. Take note of the orientation of the metal plates. After bolts are removed, apply some spray lubricant on the threads; then set aside for later use.

Remove the two top hex bolts and connecting rods.
Step 15
Remove the two metal plates, clean, and set aside.

Remove and clean the two metal plates.
Step 16
Remove gears from housing and set aside. Take note of the two rectangular nuts that are located in the housing (in the picture below, they are located in the bottom right corner).

Remove gears from housing.
Step 17
Clean housing gears and look for any damage, such as chipped teeth.

Clean housing gears.
Step 18
Remove the two motor housing bolts, apply some spray lubricant, and set them aside for later use. Take note that these are NOT the terminal posts for the battery cables.

Remove the two motor housing bolts.
Step 19
Remove motor end cap, taking note of the armatures and also the springs underneath the armatures…do not lose those springs! If your winch is a waterproof model, take note of the rubber O-ring on the end of the cap. Clean the inside of the cap as well as the terminals and set aside for later use.

Clean the end cap and terminals.
Step 20
Remove motor housing from gear housing.

Remove motor housing from gear housing.
Step 21
Remove motor from motor housing.

Remove motor from motor housing.
Step 22
Take note of the condition of the motor end which (when fully assembled) is in constant contact with the armatures. It should be a shiny and untarnished surface. In the picture below, it is dirty. If yours is dirty, clean up that contact area by using a wire brush, fine file, or fine sandpaper.

Examine the motor end.
Continued in Part 3 of the ATV Winch Cleaning and Maintenance Guide (Steps 23-32)