Usually I let a building get half or most of the way done, and then I use Slavery to spend 1 or at most 2 citizens to finish it off.
Say you have -1 due to Unhappiness. You can use 1 citizen for slavery. You now have -1 due to the "oppression" but you have 1 less population now, so you don't have the -1 due to overpopulation. So, you break even, but essentially you have your production completed for free. And, if you spend 2 to slavery, you now will almost certainly have no unhappiness penalty. (You're working one less tile, but you're now working at maximum efficiency.)
This is an oversimplification, but essentially this argument is what makes the slavery civic worthwhile.
Wodan