Command squads (and commanders) are bits gold.
Kneeling legs make any heavy weapon look awesome.
Pointing arm is great for sarges.
Bannerpole arm is cool for holding a gun straight out (like my plasma guy).
The missile launcher 'flat' hand is also popular, though mainly wiht characters (librarian usng a power, apothecary steadying an injured marine, etc)
Though with the basic tactical sprue, there's still some things you can do.
Cut the magazine off a bolter (and build the end up a little with some green stuff, or even a shaved down bit of sprue if you have;t got anythign else), place the magazine in the open 'under' hand and stick the bolter arm on with a gap between the bolter and the mag. Qucik and easy 'reloading pose'
There's also the popular 'bracing against trcoil' pose. Chop the 'under hand' at the wrist, flip it pver and stick it on top of the bolter, like the marine is holding the gun down to fight the guns tendancy to ride up with recoil.
If you have multiple squads to assemble, you could try to make them uniform. This squad is all eagle chests and beakie helmets, while this squad is all skull chests and skull forehead helms, etc. Makes it easy to tell the squads apart and makes a nice uniform look.
You could also have a marien firing sa bolter with one hand and holding a combat knife in the open hand (or an auspex for a sarge)
Grenades are goode too. I can;t remember if the tac squad comes with a grenade throwing arm, but the assautl squad does.
I like to use bits from all my squads. I like to take all my bits off the sprue and put them in the bits box and assemble from there. you;ll see tacticals with running legs, a scout sarge with a commander's chainsword arm, etc. With the exception of bolter arms, whch I leave on the sprue til I need them (the arms line up best in pairs, if you pull them off the sprue its harder top tell whatt goes with what)