In case if dehydration needs to be done slowly and under tight control of all parameters, one possibility is to use micro-dialysis buttons.
We used it for a large membrane protein complex and diffraction improved from ~7 to 2.7 A. The crystal is fished out and put into mother liquor solution in the button, sealed with dialysis membrane and the button is then placed into about 5 mls of mother liquor with slightly higher PEG concentration. Then you just exchange outside buffer every day or so for solutions containing higher concentrations of PEG. We went from ~9 to 30 % PEG4000 in about a week. You can easily observe crystal under microscope and if it cracks - you went too far/too quickly with PEG and need to use a bit less next time. Also, this method allows you to control all other components of the dehydrating solution - we needed to decrease salt concentration at the same time as increasing PEG. You can also introduce/increase cryo-protectant concentration at the same time. With these crystals, otherwise excellent dehydration machines already mentioned did not work, possibly because the process had to be really slow. The reference is here: http://www.ncbi.nlm.nih.gov/pubmed/21822288 Best wishes.
