Position relative to enemy strong points, enemy movement routes and dynamics of battle. If you can get the feel of the enemy team, what their capabilties are, where they will be, where they spawn etc you can be very effective with the MG.
Map also determines how to use the MG. Some maps I will simply not use the class at all. I use singleshot most of the time, bursts to take out larger groups or if I miss the intial shot. Suppressing the enemy is not as effective as killing them in droves. When think of suppressing the russians I think chiefly of killing so many they don't come back to that position in a hurry, which allows me to move up and gain better fields of fire.
Don't be tempted to be overly mobile unless the team is moving with you. Your job is to take out entrenched and moving targets en masse, while your team goes for the objectives. You can't quickly kill an enemy unless they're in pistol range or you picked up a rifle or SMG.