How to clean project before each build?

Press ⌥⌘R, expand the selected scheme, select Pre-actions, click +, select New Run Script Action, set Provide Build Settings from to your target. In the box below type rm -rf ${BUILT_PRODUCTS_DIR}. Note: it is BUILT not BUILD as seen in the Xcode dialog. You can type echo ${BUILT_PRODUCTS_DIR} > ~/Desktop/log.txt to see what's going to be deleted.

The selected answer did not work for me, it caused my build to fail (Xcode 4.6.3) when trying to run on the simulator.
Based on Jano's answer and on this link in the Pre-action script instead of writing

rm -rf ${BUILT_PRODUCTS_DIR}  

I wrote

touch ${BUILT_PRODUCTS_DIR}

This should have the same effect and it doesn't cause my build to fail