Seeking the kingdom of God first does not require a life lived apart from the world. Matt Perman wrote a short post about the myth (excerpt follows):

Jesus says “seek first the kingdom of God and his righteousness” (Matthew 6).

In the Middle Ages, before the Reformation, it was thought that life was divided into two areas — the “perfect life” and the “permissible life.” Those in “full time Christian service” lived the perfect life, and everyone else was relegated to second class — your life was acceptable, but not most important. If you wanted to live a truly important life, you had to be in “ministry” (which was also conceived of very differently then).

Jesus explodes this error.

He doesn’t do this by saying “the things of the world are as important as the things of God.” The teaching of the Bible is not that there are no priorities in life. Seeking the kingdom of God is the most important thing.

But the revolutionary teaching of Jesus and the Bible is that you don’t have to be a pastor or missionary or full-time Christian worker to do this.

Wherever you are, whatever your job, you can and must seek the kingdom of God first.

via The Doctrine of Vocation in the Sermon on the Mount : What’s Best Next.