Controlling the field, is taking an important area, and denying the enemy the chance to move into this space, or slow them down drastically. The most important thing with control of the field, is that now the enemy is responding to you, and has to work hard to push into the area.
Since 2/3rds of all games are objective based controlling access to these areas with troop choices has made the battlefield a new and exciting place. Area denial, and punishing those that force themselves into your kill zones is not only a ton of fun, but extremly efficient use of your forces, as what little gets into you, most of your army is able to destroy.
Here are some examples.
Drop pod walls to slow down enemy advances, followed up and reinforced by a heavy wall of armor 13. This was from a recent game of which 4 rounds were spent by the enemy trying to punch through. This was with a test blood angels army, so it was backed up by assault squads, which was able to mop up (the cleaning crew) what ever was trying to punch through the battleline I had formed. It was 8 armor 13 vehicles/dreads, 3 drop pods to form a wall, and razorbacks etc able to destroy whatever came through. The battleline was protecting the winning objective, and the enemy really had no chance to break into it.
Slowing down land raiders..... Nothing like parking chimeras/ rhinos, 1" away on front and both sides of it. Nothing says "stop quite like it" besides maybe blowing it up.
Kill zones. Ive discussed this in detail, so I wont hit it again. almost 19"'s for meltas in transports 26 1/2" for rapid fire plasma, landraider assault termies again at almost 19",etc....
Before the game starts... Infiltrators are fantastic at this. They are able to control the battlefield, and help slow down scout army lists, and other fast armies. Great distraction also.
Alpha strike, heavy gunlines. These tend to control firing lanes along their arcs of fire. Great for fire suppression, and blowing up transports to stop the enemy in their tracks. (vital now in 5th).
Unmovable mobs. The hordes move out and just swarm over objectives.
There are lots of ways to control the field, and often it just takes a little consious thought before and during your game. Each army has their own methods, as does each general have his ideas on how to do this.