Water. Some mosses can survive lengthy dry periods, recovering quickly once they get moisture again. Most mosses, however, require moisture and low sunlight in order to thrive. If your moss garden location already has moss growing, that's generally an excellent indication that the place is hospitable to all mosses. Once your moss garden is thriving, the only care it generally requires is some misting with water during any dry spell.
Shade. The majority of mosses fare better in shady conditions, or at least places where they don't face constant sun exposure. A moderate amount of sun is fine. In sunnier areas, you can plan the location of your moss garden on the shady side of a large tree. In the northern hemisphere, therefore, it's best to plant on the northern side of a tree, whereas the reverse is true in the southern hemisphere. Again, if you choose a spot that already has mosses, you'll enjoy success with a moss garden.
If part of your desired moss garden location has more sun exposure, in those spots try to plant mosses that aren't as shade-dependent. Bryum mosses, found commonly on walls or in cracks along the sidewalk, fare better than most mosses in direct sunlight. Grimmia moss is another variety to consider for patches of heavier sun exposure.
Preparing the soil. Weed the soil bed and remove any grasses. Moss will likely kill the grasses anyway, but removing any grass presence beforehand can ensure that your moss garden grows without any competition.
Transplanting moss. Many of you already have mosses growing in your yard. Lucky you - you can just cut a chunk of moss mat and then move it to the desired location for your moss garden. When you transplant a mat of moss, moisten both soil and moss mat before you plant it in place. Make sure it rests at the same level as the rest of the ground around it. Most importantly, pack the soil tightly around and beneath it.
If you have no moss in your yard already, or if you simply want other varieties to grace your moss garden, you can look for mosses at garden stores. However, mosses still aren't very commonly found for sale at these stores. But don't lose hope - look for mosses in your friends' yards and in other areas of public property where moss growth is probably undesired (like pavement cracks). You might even find what you're looking for at one of a few sites (like Moss Acres) that sell and ship chunks of common moss varieties.
You don't even need moss mats, as a matter of fact (good news when it comes time to cull moss from a crack in the pavement). Though live moss mats are the best method for transplanting moss into your moss garden, you could also just pull up some moss with your hands, throw it into a blender with some diluted buttermilk (naturally!) and then spread this cocktail over the designated soil bed. In several weeks, you should start to see the moss growing.
Instead of constantly fighting a losing battle as you attempt to preserve the health of fragile, decorative flowers and grasses, consider the mossy alternative. A moss garden might just help you reconnect with the serenity of nature.