Freezing artifact

Does anyone out there have any tricks on how to prevent/minimize freezing artefact when freezing brain biopsy tissue? We are currently using Isopentane/Liquid Nitrogen. Have also tried freezing tissue on our new Leica cryostat using the Pelletier element. Both methods have resulted in varying degrees of success and failure. Also, we use Gum Tracacanth as our freezing medium (because that's what's always been used here). Any thoughts on this would be more than welcome.