Recently I was asked about why orchids sometimes just go belly up and how to avoid it. I actually get this question a lot, and finally the light bulb blinked on - post it to the blog so everyone can read it! Silly me. So what follows is my long-winded answer. Hope you find it useful!
I maintain that orchids are not difficult, they're just different. Different rules. And the fact is that people are often exposed to discount store plants that have received poor care...which reinforces the idea that they're difficult.
The most commonly seen problem with Phalaenopsis is plant collapse or withering leaves, which typically is one of two things: (1) water stress, (2) rot disease.
(1) Water stress can be either too often or way to infrequent watering. I'm sure you can see how too little water leaves to dessicated leaves. But too frequent watering or media that stays wet too long can result in an appearance that looks the same because it can lead to the roots rotting. No roots means nothing to take in water with, so the leaves can't survive.
Most orchids, phalaenopsis included, are epiphytes, and live in areas where they camp out on trees. They get plenty of rain, but its breezy up in a tree so they dry off pretty often too. So in a pot they need a porous mix that holds air and water. It also helps to make sure they're snug in their pot (root bound), which reduces the amount of stagnant moisture in the pot.
Water well when its time to water, then let them dry out a bit before watering again. Stick your finger in the pot and feel around. Should be dry at least 2 inches down, then water well again. Also don't let them sit in water (if they're really dry, you can do that for maybe 20min, but not for a day or more like you can do with some plants). Some people put these things in what looks like regular potting soil. I personally think this is a recipe for trouble because it can stay so wet.
When you pick a plant, look for one with lots of hard white (or silver) roots with green (or purple) tips. If you grab it by the leaves, it should be stable and should bring the whole pot with it, within reason. If its freshly potted, or a slipper orchid, or in a heavy clay pot it might not, but it should still feel firmly associated with that pot.
To check for root problems, yank that thing out of the pot and look at the roots. They should be firm and kinda white. If they're dark brown and squishy, there's a problem.
(2) Other rot diseases are less common, but do happen. Again using the example of the Phalaenopsis, if water sits in the crown of the plant, this can lead to fungus or bacterial rots that will kill the center of the plant and the whole thing will collapse. How to avoid this? (a) Don't bother with misting your plants, (b) when you water, just water the media and not the leaves, (c) if you water with a hose (say in a greenhouse), make sure there's a fan that moves air about so the leaves dry off.