A good tactic with the smoke grenade to employ if you know there is an enemy on the other side of a doorway, or such, is to pull out a smoke grenade and throw it through. Let the OpFor fire off a few rounds at what they may think is the enemy coming through, then pull out a second smoke grenade and throw it through about halfway into the first smoke grenades 'burst'. If you throw the second grenade correctly it will be covered by the smoke from the first and will also push the smoke cover further and make it thicker. This confuses the enemy as the smoke is lasting longer and you can slip through/kill them before they can react to what happened.
Note: Low-End Graphics card users beware, this tactic can 'lag up' your system and drop your framerate pretty horribly. Trust me, I know from experience, owning a Low-End (but still supported) graphics card.
[WECO]Yodak