Communicate with your crystallographer. It is more often than not necessary to prepare a protein solution which contains buffers and a specific salt content to maintain the proper pH and ionic strength for a protein's stability. Detergents, particularly in the cases of membrane proteins and DNA repair enzymes, are also sometimes needed to keep a protein soluble and free from aggregation. This can be especially true during the crystallization process as the protein concentration will locally reach extremely high concentrations. So a few general rules should be followed.
Most buffers are fine.
Other compounds used for solubilization - Avoid using these if at all possible. These may oversolublize the protein. Summary Given high purity protein, at high concentrations, and enough information, one may be able to grow protein crystals which will lead to a successful and accurate solution to the protein's structure. Then you can have pretty pictures like these.