[For the first part of this entry, see below]
3. Letters must be read literarily.
I think it is a very common notion that when Paul sat down to write to the Romans or the Ephesians, for example, that he just grabbed a pen and paper and jotted down a few thoughts the same way you and I write an email or other casual correspondence. But the fact of the matter is that the New Testament epistles are all literary works in their own right, often full of many complex and intriguing elements. Passages like Philippians 2 or Colossians are thought to be actual "hymns" or poems that were no doubt sung by the early Christians. We must give care to study not only what the apostles taught, but also how they said it.
4. Letters must be read theologically.
New Testament epistles are theological to be sure, but not always in the way that we assume. When we think of theological works, I bet many of us think of some thick tome or even multiple volumes with thin pages and tiny font where one can look up just about any theological topic and read all that one would ever want to know about it. This is known as "systematic theology." New Testament epistles are not systematic theology. When one seeks to understand what specific authors are saying in specific literary works, he or she is engaging in "Biblical theology." Paul, nor any of the other apostles, never wrote an exhaustive treatment on a particular theological subject. He wrote what he felt was necessary to address the particular needs of his readers. It is the job of the systematic theologian to look at the whole of Scripture and see what all of the relevant passages teach on specific theological topics. The two methods are not opposed to each other, rather they are complimentary.
5. Letters must be read authoritatively.
When reading these letters in the New Testament, we must always remember that we are not reading merely the words of men (they are certainly that), but we must always remember that they are also the very words of God. The doctrine of inspiration teaches that God the Holy Spirit worked with a particular author's knowledge, education, experience, temperament, feelings, desires, fears, hopes, etc. to produce the word of God. This is true of the whole of Scripture ("plenary"), down to the very words ("verbal"). That Paul was acutely aware of the fact that he was writing with such authority is clear in all of his letters, especially at the opening when he almost always mentions the fact that he was an apostle. An apostle was an emissary or an ambassador that spoke on behalf of the Lord with equal authority (it is for this reason that there are no apostles today).
May you have fruitful reading!