Regarding Iraq, I believe the US needs to do a number of things to establish a democracy. The success of any attempt to impose democracy is questionable at best, but it has been done before. In Japan, for example.
Of course, Japan is a much more homogenous society. In Japan's case, the US military wrote the constitution and ensured that it was followed by maintaining forces in Japan.
I think we need to write a constition that stipulates a federal system that keeps any particular ethnic group from getting complete control of things.
There should be a legislature established through proportional representation, a system of independent courts (with its initial judges appointed by the US or a US appointed committee of Iraqis) and a WEAK executive branch. And, most importantly, a bill of rights, complete with freedom of speech and religion.
This constitution should be put to a general vote of all registered Iraqis and would be ratified with the approval of 50% or more. If 50% do not approve, it should be reworked until we get that 50%.
Perhaps the most important thing is enforcement. the government should be held to the letter of the constitution.