Usually we let it happen by waiting too long to repot, letting a poor plant suffocate its roots in a dense, mucky mess.

Of course, it's often our most valuable or favorite plant, so how do we get the plant to recover?

First, keep in mind that as a "natural creature", the plant has a survival mechanism, so it wants to recover.  Then, with that in mind, consider that our job is give it the highest likelihood of doing so.

The keys to recovery are high humidity, warm temperature, and subdued light.

Consider the following scenario:

1. A plant with no roots has no way to take up water, but can lose water through leaf stomata and cell wall permeation.  As the plant dries, it gets weaker and weaker, making it less likely to have the energy to recover.  That drying rate is directly related to the relative humidity, so high humidity sustains the plant while it tries to grow new roots.
    
2. Plant growth rates are directly related to the temperature, and warmer equals faster.  You want the plants' metabolism to be faster, in hope that it will recover before the negative effects of desiccation kick in.
    
3. Low light levels seems counter-intuitive, as plants need light to create the sugars they "burn" to function, but doing so requires water as well, further depleting the plants' internal supplies.  By keeping the plant in subdued light (not dark), you minimize that loss, again, extending the time that the plant can grow those new roots.

It may be difficult to maintain those conditions, so one thing to consider is the "Sphag-n-Bag" treatment to create a good, controlled environment.

Another option - one that adds to the probability of survival, but is not a substitute for providing the conditions above - is the application of a rooting hormone.  Brand is not critical, but freshness is (the hormones degrade with time, temperature and light exposure).  We recommend K-L-N due to its lower cost for the volume received.  The most effective treatment method for a root-free plant is a soak:

1. Fill a container with water at about 80°F (~27°C)
    
2. Add about a teaspoon of hormone concentrate per gallon of water (~1.25 ml/L)
    
3. Totally immerse the plant for about an hour.