Thermal springs , such as the geysers of Yellowstone, and cooler springs, such as those that form the tufa towers at Mono Lake, are good examples. IFor long-term preservation, over billions of years, the minerals that entomb organisms must be chemically stable and impermeable to water. The most widespread such minerals are silica, calcium carbonate, and fine grained clays. Planetary geologists do not know whether Mars has widespread deposits of silica, calcium carbonate, and clays. The Mars Global Surveyor mission will look for them, but until then we can make some educated guesses.
Carbonates are an interesting case study.
Similarly, other rocks that are rare in ancient areas on Earth, such as evaporates, may be quite common on Mars. These rocks were probably never buried or disturbed by the tectonics that constantly reshape the Earth's surface. The rocks in these ancient southern highlands of Mars have likely survived with little change. If spacecraft can find aqueous mineral deposits in those ancient highlands, they will have found the best place to hunt for fossils. After we know where to look, robots can collect and analyze samples.