The basic trick is to produce a very smooth, featureless background, preferably using low Z (atomic number) or low density materials. The following three methods exploit in 1: round glass coverslips, in 2: double stick tape and/or colloidal graphite paint for backgrounds, and in 3: a combination low Z plus high Z conductive coating strategy is described which when used in combination with 2, also produces the desired background effect.

1. I've done the following for pollen grains, rust spores, and soil particles (dried down onto glass coverslip support surface from water or ethanol, they just stick naturally), should work with larger samples, eg., insects, seeds, provided that any paint you use to mount the sample to the glass coverslip does not spread out beyond the edges of the sample so you don't see it in the background.