I think there's an unintended motivation for users to post questions to higher-traffic Stack Exchange sites rather than their more specific or appropriate lower-traffic ones. I think I have a solution.
What motivated this suggestion for me was that I am confident that if users were to post Android design questions to Graphic Design beta and/or User Experience, that they would likely not be noticed by the appropriate audience compared to the vast army of Android experts here on Stack Overflow.
There are plenty of Android questions on SO that really should be on one or both of the aforementioned sites. Presently there is little motivation for users to start questions like this on the appropriate site.
Is there something we could do to cross-pollinate related sites? For instance, what if we let certain tags span certain sites? Obviously the selection would be controlled by high-reputation users.
There could also be a feature to show (or explicitly hide) certain questions from other sites regardless of the tags. This could be controlled by lower-reputation users than the global tag since it wouldn't have much statistical impact. For instance, the android-layout tag could by default span sites, but OPs and other users could eliminate from the Graphic Design site questions that are of a purely technical nature.
Users interested in Android tagged questions would see related questions on design and UE sites--whether they are searching, posting, or viewing a question through the related items. This would help eliminate cross-site duplicates, and encourage people to post on the correct site in the first place.
For related question lists and search results, there could even be a specific cross-site icon next to an off-site question to show that it's from UX, for example.