Strategies to optimize initial crystallization hits:
Kits and screens to increase protein solubility and stability prior to crystallization as well as screens to optimize the chemical composition of your crystallization reagents, i.e. buffer pH, precipitant and additive composition.
Get your protein ready for crystallization! Consider to alter the protein surface or modify the sequence by truncation or mutagenesis. Or try a new mode of production.
Thermodynamics / Kinetics
Do not discard bad diffracting crystals! They can be very useful, too. Try to improve diffraction by varying the solvent content or use your initial crystals for seeding.