Table join order in postgres
This page in the documentation describes how to prevent the PostgreSQL optimizer from reordering joined tables, allowing you to control the order of joins yourself:
http://www.postgresql.org/docs/current/interactive/explicit-joins.html
Are you sure your table statistics are up to date? When PostgreSQLs cost based optimizer fails with such trivial things it's a pretty good sign something is seriously wrong with the table statistics. It's better to fix the root cause than to work around it by overriding the built in optimizer because the problem will inevitably pop up somewhere else as well.
Run ANALYZE on the affected tables and see if it makes PostgreSQL pick a different plan. If it still chooses something silly it would be really interesting to see the query plans. The optimizer not doing the right thing is usually considered a bug.