How do we determine which doctrines are more important and which are less important?

The answer to that question depends on how closely related to the gospel a doctrine is and how much practical impact it has on the Christian life.

1. Doctrines like the Trinity, the full deity and humanity of Christ, justification by faith alone, and the authority of Scripture are all intimately bound up with the gospel. Without them, we either lose crucial aspects of the gospel or the gospel itself. Such doctrines are therefore of utmost importance. Disagreements here divide the church from the non-church.
2. Doctrines like the proper subjects of baptism and the biblical form of church government are less closely related to the gospel, and therefore less central to the faith. Still, such doctrines are not totally unrelated to the gospel. For example, baptism is a picture of the gospel and defines the membership of the church, the people of the gospel. Other theological issues, such as our understanding of the roles of men and women in marriage and the church, have huge practical implications and are therefore very important even though they are not directly related to the gospel. Disagreements here may divide one church from another, like the differences between a Baptist and a Presbyterian. But such differences need not hinder embracing one another as fellow Christians.
3. Some doctrines, such as the meaning of the thousand years in Revelation 20, are far removed from the gospel and have little practical impact on the Christian life. This is not to say that such doctrines are entirely without importance, but disagreements here can still allow for cooperation between Christians to the highest degree, namely, fellow membership within a local church.