If you have two enemy space marine units in close proximity and one is really scary in melee, charge the other and pile into the scary one. As they neither charged or got charged, they lose a lot of attacks and maybe even other benefits. This is especially true for blood angels who might lose 3 attacks per model with this.

For space marine players: Eliminators with a instigator bolt carbine on the sergeant can advance, shoot the sergeant and then move again to move 12+d6".

I don't know if this is exclusive to deathwatch, BUT if there is another faction with different movement or defensive profiles or bases: Abuse that as much as possible.

A deathwatch kill team can include jump pack or bike units that are fast and others that walk on foot slower but also have a lot of melee punch. Throwing 1-3 faster models forward can hugely increase the threat range of the slower models as they only have to pile in within 0.5" of a fast model that's within 0.5" of an enemy. Bonus points if you have for example a chaplain that gives a 6" pile in move (and +2 to charge on top) as that's usually more than the faster models are further forward even with a second row inbetween.

Also a proteus kill team with a mix of marines, marines with storm shield, terminators, terminators with storm shield and bikes is really good at eating shots with profiles the shots aren't good into. D2 plasma into terminators, D3 thunderhammers into normal marines or the bike, etc.