Soak crystals overnight in a solution comprised of the mother liquor utilized to grow the crystals plus 0.5 to 2% v/v glutaraldehyde. Glutaraldehye cross-links proteins through the epsilon amino group of lysines. Cross-linking can be used to stabilize fragile crystals for seeding or data collection. Reference: F.A Quiocho and F.M. Richards. Proc. Natl. Acad. Sci. U.S.A. 52, 833 (1964).

Is glutaraldehyde cross-linking of crystals an effective way to protect them during soaking and/or cryopreservation?

Yes, it can work, " incredibly well". See Carol Lusty, J. Appl. Cryst. (1999). 32, 106-112 A gentle vapor-diffusion technique for cross-linking of protein crystals for cryocrystallography. It was pointed out that this method is (like so many others) can be protein crystal-dependent. Some protein crystals will effectively cross-link in 5 to 10 minutes, others require overnight. Treating the crystals for 30 minutes at 4 degrees Celsius is another approach to try. Or try 5 microliters of a 25% solution of glutaraldehyde to a Micro-Bridge in a 24 well VDX Plate or Cryschem Plate. Let the crystal cross-link through the vapor phase. Periodically remove crystals to test diffraction and see if there is any evidence that the crystal is cross-linked by physical manipulation.