I wrote a simple script that you can download here that makes it easy to update your Gentoo system. It first re-syncs your portage tree so that you are pointing to the newest source files. Then it performs a deep update including build dependencies on all packages that have new versions or could be rebuilt because you have modified your USE flags. It also upgrades any build dependencies and tools. Finally it removes unused dependencies and attempts to fix any broken packages that now have new dependencies.
emerge –update –deep –with-bdeps=y –newuse –ask world
I also found this excellent website that makes it very easy to search for Gentoo packages and see what use flags you can apply to them.