How do I downgrade packages easily with Yum?

You can use:

yum downgrade NetworkManager NetworkManager-gnome NetworkManager-glib

If you have previously had a working version on the system in question yum history is a goto command. It will list transactions and allow you to roll back to a certain point or undo single actions see more here;

http://docs.fedoraproject.org/en-US/Fedora/14/html/Software_Management_Guide/ch05s16.html

Alternatively if you wish to downgrade something that has multiple version restricted dependencies using the yum-allow-downgrade plugin will sort out the dependency errors you are seeing. It will add a --allow-downgrade option that you can use.

If you are deliberately sticking with an old version I would suggest looking into the version lock package. This will prevent yourself/colleagues/scheduled updates ruining your work.

http://docs.fedoraproject.org/en-US/Fedora/14/html/Software_Management_Guide/ch06s25.html