I have found moss to be a challenge to cultivate, and thought the forum would be a great place to determine what works and what does not. To begin with, there are many different types of moss, and they all have their own desired environment. Most of this discussion on moss will relate to silver tip moss, although most of the mosses that exist in our high arid desert conditions will follow the same protocols.
The first major hurdle to overcome is maintaining enough moisture at the soil surface to maintain moss growth. Using a top dress of shredded sphagnum moss helps to achieve the desired moisture retention.
Most plants tend to photosynthesize best at a temperature of 76 degrees fahrenheit, while moss seems to produce its best growth around 50 degrees fahrenheit. For this reason I believe the early spring is the best time to get a jump on moss growth.
The third issue generally seen is that moss prefers a pH of 5-6 where as most of our bonsai prefer a pH of 6-7. To make things more difficult water from an aquifer typically has a pH of 8-9, and Denver water has just changed their water parameters to a pH of ~8.
The fouth parameter to be aware of is the total amount of dissolved solids (TDS) that are in the water being used. My tap water tests at about 700 TDS and has proven to not grow moss well if at all. I have been using a reverse osmosis filter to reduce the TDS to 4, and then using a fertilizer made for RO water, which brings the TDS back up to 200. For the past 3 months the water with a TDS of 200 has proven to work great for keeping the moss vibrant and alive.
Theses are a few of the parameters I have been able to determine thus far, and I hope will will share some of the tips and research you have used to successfully grow moss for your bonsai.