Here's a small (and perhaps obvious) trick I sometimes use with factory_girl.
You may find yourself doing some complex or recurring setup in tests.
That complexity doesn't really belong in the tests themselves, but it's also difficult to model in factory_girl's definition language. Perhaps you have tricky callbacks or dependencies across deep object graphs that are hard to get rid of.
Then make a plain Ruby method alongside your factory definitions:
factory :item do
title "Default title"
item = FactoryGirl.create(:item, attributes)
Now you can just do
FactoryGirl.create_item_with_complex_stuff(title: "My item") in any test.
Since the method is in your factory files, it's easy to find alongside your other factory definitions. The complexity no longer clutters up your tests, and you have all the power of Ruby at your disposal.