Branch by abstraction: Are there "examples" of how it can be done?

Question

Having read Martin Fowlers "Feature Branch" and Flickrs "Flipping Out" I guess there are a few guys out there who do:

• all (or most) development on Trunk
• release Trunk regularly (assuming updating your web site)
• not-yet-approved or not-yet-finished features should not be visible/have no impact on the regular user

I've got 2 questions:

1. granted - Flickr's article seems to work for "frontend code". But how is it cleaned up? Don't the ifs pile up?
2. how does this work for the more "backend part"? Thinking of database changes, or model refactoring. Working with ifs doesn't seem to work - and copy-pasting classes for small adaptions also seems awkward.

Are there any articles out there answering these 2 questions?