On my trip to Trieste, I picked up a copy of the Nuovo Zingarelli Minore, an Italian language dictionary, by Nicola Zingarelli. Not a translation dictionary, but a standard descriptive dictionary entirely in Italian. Zingarelli is the most popular dictionary in Italy and is widely used in school and work. If your Italian (or any other language for that matter) is at the stage where you are reasonably fluent, or at least have a sufficiently large vocabulary, why not consider using a descriptive dictionary instead of a translation one? It forces you to think entirely in Italian when you encounter a new word, rather than translating it into English in order to know its meaning.
The Zingarelli dictionary is well laid out and complete. One of my favourite features is the “Sfumature di significato” section below certain words. These sections explain the differences in meaning between words that are sometimes confused or that seem to have the same meaning. There are also hundreds of clear illustrations of objects such as tools, pieces of furniture, houses, and, this being Italy, building styles.
Another brilliant feature of the dictionary is the set of large pictures giving the names for common objects, animals, plants, anatomical parts, amongst others. There are also useful (and fun!) sections on Italian and Latin proverbs, male and female names, and even names for people from different towns and cities in Italy.
If you can get a hold of this dictionary, it will be a great companion in helping improve your Italian.