This question is difficult to answer and you should be more specific.

Do you have covered slides or "swimming" material?

In the first case you have to "dip" the material in a solution of a fungicide or fungistaticum before immersing the samples in glycerine and covering them. In the second case you can add thymol to the glycerine (but in which concentration I can't remember). For other organic samples (melanosclerite or diatoms) sometimes ethanol or methanol are appropriate, but this can result in problems if pyrite/markasite is involved (even if it is fine distributed within your samples).

You may ask additionally Prof. Dr. Hans Kerp from University of Münster. He is a palaeobotanist and experienced in this question.

We have the "swiming material" and thymol is probably the best solution. Thank you.

I used touse a few crytals of phenol. I presume you keep them in a fridge.