Be sure to use coasters and to immediately clean up any spills with haste. Standing moisture or harmful chemicals can damage wood surfaces!
For cleaning, use a solution of water and mild soap or vinegar (half a cup of vinegar per gallon of water). Thoroughly dry the block.
You can remove minor scratches using steel wool or a fine-grit (#220) sandpaper. Be sure to sand with the grain of the block.
If you wish to apply a touch-up finish, remove all dust and residue before applying a 45-sheen polyurethane similar to this. Always test in an inconspicuous area first, to make sure the poly matches the original finish.
For deeper scratches, tape off the area to be repaired, spray the finish onto a rag and rub the finish in. If you are repairing a deep scratch or sealing bare wood or cut edges, use a brush to apply a finish similar to this.