To be honest, I had a hard time getting started on this post, which means I'm most definitely phlegmatic! My melancholic half mulled over in my mind what I wanted to say in this post, and my trusty phlegmatic other half thought, "Eh… I'll write it later."
The phlegmatic temperament is difficult to write about because this temperament almost seems "non-human." The detached, mellow, almost "hyposensitive" nature of this temperament seems better suited to a robot than a human. In fact, at least one spiritual writer I know of even questions the existence of this temperament.  However, all of us know people who fit the description of this temperament very well, so I think it's legitimate to write about it.
I think it's good to follow up the post on the sanguine temperament with a post on the phlegmatic temperament because the "cure" for both of these temperaments is pretty much the same. I like to think of the phlegmatic as a "sanguine on Lortab." The phlegmatic is generally friendly, values people over principles, and struggles with the same lack of follow-through as the sanguine, but without the "intense" personality one sees in a sanguine.
The primary problem for the phlegmatic is one of motivation
. While a sanguine will be easy to motivate and act but soon lose interest, the phlegmatic has a hard time moving beyond interest to actual action. Though he will be dedicated to any project, he needs a little prodding.
The most important thing a phlegmatic can acquire to get him on the path to heaven is a regimen
, especially a regimen of prayer. Much like the sanguine, he needs a structure that will keep him on the straight-and-narrow so that he will not become complacent in the spiritual life. Unlike the sanguine, however, he will actually enjoy
this kind of structure. Structured prayers such as the Rosary or the Divine Office are tailor-made for phlegmatics. The only danger in these types of prayers for the phlegmatic is that he may become so absorbed with the routine that he stops growing. He needs to employ the imagination in his prayer life to avoid this kind of pitfall.  He needs to seek what God is asking of him in his prayer and meditation, otherwise the prayers will just become rote and no change of heart will result.
Along with a regimen of prayer comes the necessary accountability
that will ensure a successful outcome. The phlegmatic needs to be accountable to others to keep him going. He also needs the guidance of others to develop a properly formed conscience and understanding. Like the sanguine, his placement of people over principles in the hierarchy of truths can lead him to become a "crowd-pleaser." He needs accountability to a spiritual director, prayer group or any other system outside of himself that will help pull him more toward the center.
Also, though he naturally hates conflict, he needs to learn to "let his 'yes' mean 'yes', and his 'no' mean 'no.'" Otherwise he will end up doing things simply to please others or to "shut them up" which is a sure recipe for resentment. Being accountable to others in his life decisions will help make sure this doesn't happen.
The great natural virtue that accompanies this temperament is "meekness." Almost everyone loves a phlegmatic. Their peacefulness and even-keel makes them great team players. With a little discipline, those of the phlegmatic temperament can find themselves on the fast-track to sanctity with little of the "inner resistance" that impedes the other temperaments.
 Fr. Joseph Massman in his book "Nervousness, Temperament and the Soul"
writes the following: "If a more exact determination of temperament is to be made, one has first to observe the basic disposition
and then the individual variation
and the excitability
of his emotions. On careful examination of his basic disposition, the phlegmatic person will turn out to be shallow, cool and perhaps even hyposensitive. He will be distinguishable from the sanguine man only in coloration and in his excitability, and will thus represent no distinct temperament
. I therefore regard the expression "phlegmatic, as denoting a temperament, as unnecessary" (emphases mine).
First, Fr. Massman makes clear in his introduction to his book that he is not writing from a scientific perspective, so his opinion isn't set in stone per se. Second, I think the simple, time-tested observation of people has made clear that some people who fit the classical description of the phlegmatic temperament simply don't possess "sanguine" traits such as "flightiness" "superficiality" and "extroversion" which are typically and unmistakably sanguine. On the contrary, phlegmatics are often very deep, loyal and introverted. Therefore, I think Fr. Massman is wrong on this one, and that the classical understanding of four primary temperaments as presented by the Bennett's is the correct one.
 (Bennett, 246)