This subject is poorly documented and a bit of a "black art", however Ive found a Microsoft quick ref card here.
Note Outlook behaves differently if the library is greater than 500Mb. If less then it sync everything, otherwise it syncs only headers and you have to manually sync documents. I cant find any way to alter this functionality.
I also find some interesting training slides from TechNev on the subject.
Microsoft's office page for this subject is here