What conditions increase the chances of fossil formation?
Rapid burial in a marine environment where there is a lot of calcium carbonate to act as cement and other minerals.
Rapid burial under large amounts of sediments prevents the decay of the organic material. A cast or mold of the " footprints" shell or other evidence like impressions of even leafs can be preserved if buried rapidly enough.
Once the evidence of the living organism has been buried it must be preserved. Hard parts like bones are the most likely to be preserved. It is much harder to preserve soft parts or the organism.
The presence of Calcium carbonate which can come from broken and dissolved sea shells acts as a preservative. Calcium Carbonate is cement. The calcium carbonate can rapidly turn into hard rock preserving the fossil remains. Other minerals can replace the organic material atom by atom as in fossilized wood, preserving the details of the once living matter.
Fossilized spark plugs are somewhat common on the Oregon coast where mud slides heavily laded with minerals cover the evidence of living things. The rapid burial and presence of minerals preserve the hard parts. To be later revealed by erosion and wave action.
The conditions for the best preservation of fossils is rapid burial in sediments that contain high levels of minerals.