I have not been DMing long, the game I have here is the only one I have DM'd in (and I have only played a couple of 'sessions' total across all the games I have been in) so role-playing or strategizing as the monsters is tough for me.
http://themonstersknow.com has been a useful resource for me, giving me insight into tactics monsters or groups of monsters would use. This guy has covered a lot of different types from the official 5e books (MM, VGtM, MToF) and I can't recommend it enough!