The setup of a battle must not be overlooked. If you have a lazy GM or module, you end up with a bare room to fight in, a lot. Then this theme isn’t for you. A good encounter has difficult terrain, places to hide, concealment, and maybe a lava pit or three.
Controlling the Field With Spells
The most obvious way to gain terrain advantage is to make it yourself. Use a grease spell to trip up enemies coming through a door or force them to walk around something and open themselves to flanking. Think about what can give you and your teammates the biggest advantage, and be aware of your teammates’ strengths and weaknesses–perhaps your monk can waltz freely through your web spell while the enemies are trapped helplessly.
Around a Corner
This is the best position to be in with a ranged weapon like a bow. Because the around-a-corner rules work differently for melee and ranged weapons, you can gain +4 cover (negating your attack of opportunity provocation!), but not give your enemy such a penalty. This is amazing! Take advantage.
Push into Dangers
You can Bull Rush opponents off of cliffs and into spiky pits, sure, but not every encounter has those do they? Well, there are other hazards and other ways of manipulating opponents. Position yourself such that you flank, and moving out of that spot puts the enemy within reach of an ally. Place yourself so you limit an enemy’s movement options, or put them in a position where they will be in trouble in a moment (coordinate those fireballs to win!).
When you are yourself limited in movement options, think about alternate ways of getting through an obstacle. If you can’t tumble through an enemy’s square, perhaps you can run away and draw them into a more open zone. Perhaps you can run over the top of the bookcases in the library rather than between them.
Plan the Opportunity Attacks
Most of the time, you will face opponents who lack the Combat Reflexes feat. Rather than everyone taking up an inferior position because they need to avoid an attack of opportunity, plan to provoke one on purpose with the heavily-armored fighter so an enemy can’t hinder your movement nearly as much.
I hope some of these tips help you out! These are the tricks good players use. Go forth and control the battlefield.