The Standard Tactic You Should Employ Is: Make The Game Fun.
This may sound like a trite answer, but it's really important. As a DM, you don't need tactics to "beat" your players, because you control more than just the monsters' tactics: you control which monsters make up the encounter, what the monsters' stats are, the layout of the map, the nature of environmental features, which events occur during the fight, etc.
As a DM, think of yourself more of a movie director than a military tactician: your goal is to orchestrate entertainment rather than victory. As a result, the tactics you should employ for monsters should reflect the atmosphere of the battle.
Here are a few tips I've found effective when controlling monsters in battle:
Games are a series of interesting decisions. -Sid Meier
No matter what tactics you employ, make sure they give players choices in how to respond to them. The worst thing about using good tactics is that they often leave players with few choices in how to respond. The challenge in the fight should come from the monsters and the environment, more than the tactics they employ. Having a monster take up a disadvantageous position encourages players to exploit it. Having several monsters take up disadvantageous positions gives players an interesting decision: which do I exploit first?
Make the players feel like movie heroes.
Movie heroes often catch the enemy in a disorganized state, or manage to easily outflank them, or set up overly complicated combination attacks that wipe out entire armies. Find ways to set up your monsters that give players opportunities to create moments of cinematic awesomeness. Have a monster step dangerously close to a pit, knowing a player will try to push him in (the player still has to succeed on the roll!). Stuff like that.
Don't block players in the doorway.
This is key, and straight from the DMG. Monsters often win initiative rolls, and DMs often charge those monsters up to the players, and this often leaves the players stuck in the entry of the room. The rest of the battle plays out in the same half-dozen tiles the party began on, and all that work building the battle map is wasted. Do whatever it takes to spread the battle throughout the room. Have monsters hide around obstacles to lure players in. Go up and grab them and throw them into the room if you have to.
Move your Monsters, even if it means incurring opportunity attacks.
The DMGs can't stress this enough: if you don't move your monsters, your players won't move either, and everything ends up being a boring old stalemate. The fact is, OAs are fun and a lot of player powers and feats are built around taking opportunity attacks. A Warlord's Viper Strike is pretty boring if the DM never incurs OAs.
Spread out the damage.
It can be tempting to focus all attacks on the Defender, especially if he's marked all your monsters. Or, if one player has exposed himself, it could be tempting to pile everything on and pound him into the ground. In reality, it's more fun to spread the attacks around, it gives every player a chance to be hit, and it makes life easier on the healers if damage isn't quite so spikey. As a secret DM tactic, you may want to watch the amount of surges each character has and try to smooth damage across those values to keep things dropping evenly: if you have to end your adventuring day because your defender has no surges left and everyone else is full, that's boring.
Don't be afraid to waste monster HP
It's my experience that most monsters have plenty of extra hit points, and if you play monsters conservatively it just drags out the battle. When in doubt, give players more monsters to fight rather than make monsters more tactical. Trust me, your players will have a lot more fun flinging out big AOE attacks on clumped up monsters and blowing huge chunks of HP away than they will chipping down a few crafty and well-positioned soldiers. Plus, would you rather kill 3 monsters with sneak attacks while flanking them, or kill 1 monster with regular attacks because you couldn't flank? Find ways to let your players use their powers to their fullest potential, even if that means employing bad tactics.
Roleplay the tactics
If the encounter group has a leader and some means of communicating, have them verbally shout orders that the players can overhear (if they speak that language) and have the other monsters follow them. You might think "that's stupid, the players will effectively counter my orders and my monsters will be wasted." Rather, that's a good thing! Players usually enjoy thwarting the plans of their enemies!
A good encounter will have terrain that is both advantageous and disadvantageous. Have some monsters use some of the advantageous terrain, and have some monsters try to move players into disadvantageous terrain. At the same time, don't use is all: give the players the opportunity to find and use some for themselves. Let them feel like they out-smarted their foes.
Scare your players, don't frustrate them
One of the major changes in later Monster Manuals was the reduction in debilitating powers. I remember fighting a group of Krenshers and they kept using their daze (save ends) fear attack, and that just sucked. Turn after turn, we just had one thing to do. It wasn't scary though, because it was so predictable. On the other hand, we had fun against archers that shot past the defenders and restrained the controller and leader with pinning arrows, then skirmishers ran past the defenders to harass them. The squishies in the back were freaking out and everyone scrambled to save them. It was fun, and everyone got to (had to?) wield all their powers to save the day.
Don't conserve powers.
This is another one straight from the DMG. If your monsters have limited use encounter powers, use them as soon as it makes sense to do so. Don't save your AOE if it only hits 2 or 3 heroes: use it! A coordinated assault by players can drop a monster surprisingly fast, so it's better to use an encounter power poorly than lose it.
Don't think too hard. Act impulsively.
As a DM, you've got a lot to worry about, and a lot of monsters to control. Often, the first idea that pops into your head is good enough. Use monsters to elicit excitement and surprise! Do unpredictable things too! Do crazy things! Don't just reward players' creativity, reward your own!
It's quite simple: sometimes there's nothing really good for a monster to do when its turn comes up. No problem: ready a simple response, such as taking a ranged attack on the first thing it can see or charging the first enemy it can reach. Readied actions can also give players fun opportunities for role-playing, like if they know several archers all have bows trained on them and they have to pick the sacrifice to go out and trigger all those shots...
With all this in mind, I also recognize that some gaming groups are much more interested in hard-core tactical combat simulation than others. In this case, I'm not sure how much general advice is relevant. Many monsters, especially higher level ones, have very unique powers that lead to them being used in very different ways. Moreover, the combination of unique monsters and unique maps can lead to very different strategies. DM'ing a tactical wargame is a very different experience from DM'ing a more cinematic RPG. You'll probably want to think out the tactics the monsters will use when designing each encounter, both map and monster composition, and ensure there is some fair but challenging synchronicity between the two.