Rely on the senses. When writing, it is common to only describe what the reader should see in their mind's eye. However, to really give the reader a sense of what action is taking place or what a character is like, think about how each sense comes into play. An activity that can help stimulate the senses is to imagine that you have never seen a familiar object before --- it could be something like a ball, a piece of candy or an item of clothing. Describe it as if you are experiencing it for the first time.
Use photos or comic book panels to offer visuals that will inspire detailed stories. If the photo has captions or the comic has a word bubble, remove them and consider what is actually happening in the photo or drawing. Detail who or what is in the photo, why it might have been taken, where it was taken, the colors in the visual -- any information you think can be associated. If this is done in a classroom setting it could be very interesting to see the number of different takes each student has from the piece.
Do not confuse being verbose with detail. Sometimes the most amount of detail can be delivered in more concise ways. A good activity for learning the difference between useful description and excess wording would be to skim through a piece of work, yours or an excerpt from a book, and trim wherever possible. Consider what information you lose or what impact you gain during this exercise and which piece you remove or change. Something as simple as changing "there were many" to "987 people" can make a large impact in the quality of the writing while adding better detail.