So, you know you are going to be getting 20 or so new species this season, right?! A good idea is to mix your medium now. Why wait? After rinsing the sand (and hopefully the peat) I fill my pots with the mix, and I leave these exposed to the elements to further leach: the longer they get rained on the better. The rain continues to leach the nutrients out of the pots. When I use half of the pots I made, I make up some new ones. Time and weather are good friends to CPers, and if the pots stay around all season before getting used , so much the better. I see so many posts on all the various forums regarding the infection of fungi and mosses, and initial micronutrient contamination is most often the cause of this. This simple plan will aid in the removal of these nutrients.
Where do the nutrients come from in the first place? Sphagnum environments DO have some decomposing bacteria and fungi, so there are trace nutriens present in the peat right from the start. Sand quality varies widely, and is often coated with other mineral dust. It is the main source for mineral contamination. Rain water is also to blame, esp. if you are taking it from a storage barrell: algae, and nitrogen fixing bacteria take up residence in these storage containers if they are not kept in the dark. Once in the pot, they get to work pulling nitrogen from the atmosphere and fixing it into the medium, living off of the trace nutrients present in the mix. Mosses then take advantage of the available nitrogen, grow, die and begin to be decomposed by the bacteria and fungi, continuing the cycle that we are trying to avoid in our CP cultivation. This scenario is fine for most plants: it is a part of normal good growth, but for CP it is slow but certain death.
Occasional top watering with pure water, or even leaving the pots out in a gentle rain is a good housekeeping measure. A topdressing of live sphagnum is likewise a good indicator as to potential problems in the mix: the moss is very sensitive to salt accumulations and will soon let you know if something isn't right.
In short, look at your pots! Got moss? Got mildew or fungus? Got slime? If you answer yes to any of these, you have some work to do :-) Just as when making beer or wine, cleanlieness IS an issue.
Don't think I am immune! Despite my best efforts, things can still go wrong, and do. If after giving it my best go, if the problem persists, I repot into fresh medium: it's as simple as going out back and grabbing a premixed pot, rinsing off the roots of the plant and plunking the plant back into a happier, safer home. You can only kid yourself for so long, either the problem is fixed or..... I like to do transplants right after a good rain: the pots are premoistened then!
Hope this helps!