I can relate to you, since we will share two citizenships soon.
(1) You HAVE to use your US passport to re-enter the US
(2) You HAVE to be carrying a US passport on you when you depart from the US
(3) You are free to use EITHER a US or foreign passport when travelling abroad
Now as a Canadian citizen, I will always present myself as a Canadian citizen to a Canadian official. Only Canadian citizens are guaranteed entry into Canada. Similarly I will do the same when visiting Pakistan, even though Pakistan will let me enter using a Canadian or US passport. Hence the following is my "algorithm" for traveling:-
(1) US passport to enter the US
(2) Canadian passport to enter Canada
(3) Pakistani passport to enter Pakistan
(4) For any other country, pick whichever passport is most convenient (visa free entry, lowest visa fees, relations between host and passport country, etc)
What should one do when travelling to countries other than one's native one?