As with most games, to play Zombie Panic: Source efficiently you're going to need to know some strategies to aid you and your team when in combat, exploring, or preparing things during the course of the game. Some of the basic things people can always improve upon are surviving, playing as the undead, and barricading among other things.
A basic understanding and familiarity with the maps, stamina, weight system, guns, and melee weapons also helps greatly with improving your overall effectiveness as a player in Zombie Panic: Source.
Also note that the map pages have their own strategy sections in them for both survivors and zombies if there is something map-specific you are looking for.
For a more basic introduction to Zombie Panic: Source, see the manual.