Older glass is often found with cloudiness or etching caused by chemicals in dishwashing detergent or by water or alcohol standing in vases or decanters over a long period of time.  I've had some success removing etching with these methods:

• 1.  Fill the item with hot water, drop in a denture cleaning tablet, and let stand for half an hour or so.  Use a bottle brush to clean the inside of the item.  If the inside is too small or too deep to use a bottle brush, drop a tablespoon or two of uncooked rice into the item and swish it around to "scrub" the interior.
• 2.  Wet your fingers with water and dip into a bowl of baking soda.  Rub interior and exterior of glass item and rinse.  This works very well for light dishwasher etching.
• 3.  Use a strong cleaner like Goof-Off (not the orange-scented version).
• 4.  Use a metal polish like Simichrome.  This works surprisingly well on light etching if the shape of the piece allows you to get to the etched part of the item.

These remedies don't always work.  If the etching is too serious, sometimes there is nothing that can be done to cure "sick" glass.  If all else fails, displaying the piece with water inside (with or without food coloring added) makes the etching less obvious.

Addendum - An ebay user who read this guide sent me this information that might be helpful:  "I have another trick for displaying cloudy glass like an old perfume.  I have found that swirling baby oil inside not only makes it look clear again but, unlike water, it doesn't evaporate and leave more marks on the glass!"

Here are some of my current glassware items.