There are a couple of ways to initiate the formation of crystals. These methods can be used to avert the formation of oil or prompt crystallization if it doesn't occur spontaneously. The first thing to try is 'scratching' or rubbing the flask with a glass rod or metal spatula. A second tactic is the use of seed crystals. Taking just a few crystals from the previous crop and putting them in your solution will promote the growth of more crystals. Often, it is enough just to stir your solution with a spatula that has touched some of the desired crystals.