There are at least three dead-end approaches to decision making that I see in the evangelical world. And there's only one approach that fits with what we theoretically believe about God, life, the universe and everything.
The first dead-end is pragmatism. Pragmatism says "we should do X, because X will work". It crops up all over evangelicalism, often in the form of imitation - it worked over there/for them/in the 16th century, therefore it will work here and now. Who you imitate will depend on what evangelical tribe you belong to, but it doesn't really matter whether you're copying Calvin or the church up the road. When pragmatism isn't imitation, it is usually sparked off by one person's "big idea". That person is usually an activist - pragmatism appeals to activists. But the key thing is that for the pragmatist, the right thing to do is what works.
Problem: as you are not God, you do not know the end from the beginning. You don't know what will work. You cannot ascertain what the results will be. So how can you decide what to do?
The second dead-end is mysticism. Mysticism says "we should do X, because God told me so", or perhaps "I feel led to do X, therefore I should do it". Mysticism is rampant amongst evangelicals of all flavours. Sometimes it is just a cover for something else - there may be pragmatism lurking under there - but often it is a genuine feeling that the Spirit is pressing us in a particular direction. And I don't what to knock that entirely; I know that sometimes God guides in this way. But there is a...
Problem: mysticism is entirely subjective, and therefore inherently individualistic. If you feel led to do something, how can I critique your "leading"? What if I think you're wrong, on other grounds? Can we even have a conversation about the decision any more?
The third dead-end is biblicism. Biblicism says "we should do X, because it says so in Leviticus 18:4". (Don't look it up, I have no idea what it says). Biblicism is popular in evangelical circles because it seems to show right regard for the Bible, and because it acknowledges what we all know to be true: that Scripture should guide us. But often a veneer of biblicism is added to pragmatism or mysticism - we decide what will work, or feel a sense of leading, and then find a Bible verse that matches up. Even when this isn't the case, biblicism tends to basically mean taking one verse or passage out of context and basing my decision on it.
Problem: the Bible is not like a horoscope, where I can dip in and out for personal guidance. It doesn't function that way because it isn't meant to, and when it is used that way it is being used illegitimately. You can sanction almost anything with a bit of biblicism.
The only genuine help with decision making is theology. Theology looks at the whole scope of the Bible's witness to Christ, and tries to ascertain the character and shape of what God has done and is doing in the world. The good theologian will seek to meditate on the whole plan of redemption as it unfolds in Scripture. The good theologian will look to understand the Bible as a whole, with all its apparent difficulties and paradoxes, and that will mean understanding it as a unitary (though diverse) witness to Christ.
Then, and only then, the theologian looks to the situation in which he finds himself, and asks "what decision fits with the programme outlined in Scripture? What decision rings true with the overall direction of the drama of Scripture? What decision tends to make true, out there in the world, what is true in the pages of the Bible?"
We should do X, because the gospel applied to this situation means X.
Of course, that is much harder work. We will need to be continually steeping ourselves in Holy Scripture, because we surely won't have time to do all the work just before we make a huge decision. And we will need to be in conversation with others who can pick up our blind-spots and show us things about Christ we would have missed.
But this is ultimately the only God-honouring way to make a decision.