This may not be the answer you expected to read when you saw the title of this post, but it is oh-so true, oh-so needed, and oh-so doable:
The source of all our theology must be the word of God, and so the most obvious but often overlooked way to develop our theology is to read broadly and thoughtfully in the Bible. If you want to improve your systematic theology, ensure that you are regularly reading through the Bible and being constantly exposed to God's thinking about subjects that you would otherwise avoid.
--Phillip Jensen and Paul Grimmond,
The Archer and the Arrow