From CCP4 wiki
There are a number of methods which can be used to modify the crystallization behavior of proteins.
=== Genetic/Mutagenesis Approach===
There is a webserver, called SER (Surface Entropy Reduction Prediction Server), which aims to predict sites that are most suitable for mutation designed to enhance crystallizability:
=== Chemical Modification===
A recent study (Walter et al.  "Lysine Methylation as a Routine Rescue Strategy for Protein Crystallization" Structure 14:1617–1622) demonstrated that, out of 10 non-crystallizable proteins,
4 proteins became crystallisable after reductive methylation.
Here is a protocol for [[Lysine Methylation]].