The major downside to using polyurethane varnish is that not all varieties clean up with soap and water. This can result in a messy cleanup process. But if you prepare in advance, you can make it as easy as possible.
Step 1 - Read the Can
The back of the polyurethane can will tell you exactly what to use to clean it up, usually, it will be mineral spirits or paint thinner. Make sure you have some.
Step 2 - Prepare
You should probably not clean up right in your kitchen sink (although you can if you so choose). In order to clean up, you will need a solvent, rags, hot water, soap, and hand lotion.
Also, make sure you have protection for any painted or varnished surfaces that the solvent may come in contact with.
Step 3 - Apply the Solvent
When you are ready to clean up and your hands are sticky with polyurethane, you'll need to cut it with the solvent. Apply solvent liberally to your hands, and then use your shop rags or paper towels to thoroughly and methodically wipe down each part of your hands.
Soap your hands with comfortably hot water, lathering well.
Step 5 - Repeat If Necessary
You may still find some sticky spots you missed. Dry your hands and repeat the previous steps.
Step 6 - Clean Up
Dispose of towels or rags appropriately.
Step 7 - Moisturize
Solvents are very drying to the skin. Apply hand lotion liberally.