First, we suggest prevention:

1. Use coasters or placemats for your hot dishes and glasses. 
2. Keep a quality furniture finishing paste wax like Briwax or Liberon Paste Wax handy. Available at hardware stores, the wax should be applied to your surfaces every few months to form an invisible wax shield between moisture and your finish. Applying wax is quite easy. Take a damp rag, dip in the wax and lightly cover your table or surface in wax. Allow to sit for about 10 minutes and buff or polish off in the direction of the wood grain for a smooth finish with a sheen.
3. Wipe spills and rings immediately to keep from absorbing into the finish.

If prevention is too late, here are some simple ways to remove the ring:

1. Keep Jasco White Ring Remover or Liberon White Ring Remover handy.  Available at most hardware stores.  Take the rag and rub it on the ring until it’s gone. (this handy rag is reusable, store and save for future accidents)
2. Put some mayonnaise or other oily food on the ring for about 15 minutes and wipe off. Other good oily food options: olive oil or peanut butter.
3. Heat the ring out.  We suggest this at last resort, because there is room for error, be careful not to scorch your table!  Find a fine white cloth like a handkerchief and place it on the ring. Take a hot iron on a DRY (no steam setting) and lightly iron out the stain with quick, feathering motions on the handkerchief only. You are essentially gently drying the moisture out with the iron and handkerchief.

Once your ring is gone, apply a layer of paste wax to the surface (see prevention above for how). The paste wax will smooth the treated area and add a layer of protection to your table top.