It is vitally important to keep an eye on what the other human players are doing. A handy tool for this is the Army Report in the Reports Menu button. This report will show the location of all armies in the game at any given time. Learn to check this report frequently during a turn. It allows you to see if anyone is sending armies toward your cities or your heroes.
Try to get to as many ruins as possible. There's only a finite number on each map and you shouldn't allow your opponents to gain all the magical items and allies that these ruins hold.
If you're playing a game with Hidden Map turned on, one of your first priorities should be to scout the entire map.
Some human players like to raze cities instead of capturing them. The best tactic against these players is to fight fire with fire. Once you start razing their home cities, it will force them to hold at least a few cities. They will also be in a worse position as newly captured cities usually have weaker defenses and no production. You could also just turn off the razing cities option.
Flying units are extremely powerful in a multi-player game. Even a Giant Bat can cause a fair amount of damage if an opponent is foolish enough to leave his home cities undefended.
The Invisibility spell is incredibly powerful for obvious reasons. A deadly tactic is to have your hero cast an Invisibility spell and send him with an entire stack of flying creatures toward your enemy's rear.
Finally, the ability to create a custom army side for multi-player games opens up much room for debate as to what would make up the ultimate side. Individual preferences vary, but this is what we prefer: Our regulars consist of a Giant Bat (not only for flying, but for view as well), Dwarf Crossbows, Pegasi, Knight Lords and Dragons. You get this group on a roll and it can be a lot of fun. Then, you'll take the "No Mercenaries" option, with your Allies being Pegasi, Air Elementals, Dragons and Dust Wyrms. You should have a Barge and your heroes should be a General, Paladin, Priest, and Wizard. You want an eight unit stack, turn "sell items" off, and start everyone with 172gp and a max of 13 Grey Mana.