There are a few ways to fix scratches and other mars on wood furniture that are relatively simple.

[edit] Steps

Try cigarette ash. It is very fine and will not scratch. It can be used to smooth a rough area or to remove water marks on wood finishes.
Test nutmeats. For example, walnuts work well. Using a fresh nutmeat, break it to bring the oils up and rub it into any fine scratches on wood furniture.
Solve deeper scratches with magic markers. These are great for coloring in deeper scratches on wood furniture. Use indelible ink pens, choose a color a bit darker than the wood.
Fix dents. For dents in wood, place a few drops of water in the dent, allow that to sit for about an hour. Then using a damp tea towel and a steam iron, go over the area several times until it is warm and swells up.
Clear up cracks. For cracks in wood, use mayonnaise - this will heal deep and long cracks in wood furniture. Layer it on thickly over the crack and into the crack. Let it set several days. Wipe it down. Repeat the process if necessary. (I have healed a ½ inch crack with this method.)
Mend warped wood. Remove the warped piece if possible. Place it with the curve high in the middle. Cover the wood with a damp cloth and place a light weight on top. Continue this process for several days repeating steps as necessary. As the wood straightens, more weight can be added. Do not add weight daily or it may cause the piece to crack and become useless or ruined.

[edit] Tips

Some of the methods above may need to be repeated several times to get the desired results.
Please use the tips with some caution. Try the simplest methods first and if the results aren't desirable, try another method.