In 40K everything has a variety of things it can do - some obvious, some not so obvious. Whether you're building a list or in the middle of a game, keeping the toolbox of options and abilities in your mind can help.
Most people have at least some idea of what they're going to use their units for when they build a list. This unit is to hold an objective, that one is to bust open tanks, that one is to assault stuff, and so on. But in many cases they're just looking at the top section of the toolbox, the commonly-used screwdrivers, pliers and the like. Pull that top section out and root around in the bottom and see what you might be able to do with the Allen wrenches or an angled nut driver.
Just flip through the rulebook and find a section to look over. Is there something there you may be able to make use of? Do you have a way to counter someone else doing so?
Take a few simple, but fundamental examples.
- Line of sight (LOS). If someone has something behind LOS blocking terrain that you have to take out, do you have a way to do so? Perhaps something fast enough to get to it quickly, a different deployment type like Deep Strike or Snikrot, or a way to hit them regardless like indirect fire? On the flip side, are you taking advantage of LOS, protecting your units where possible?
- Movement. While most things in 40K can move, many players only see movement as a means to one of their own ends - getting into range to shoot or assault, or getting to an objective. But it can also be used to deny the enemy from achieving their ends by doing things like blocking access to objectives or getting throwaway units in the way of desired assaults. Your units aren't just a collection of shooting and close combat attacks; they're also little bits of partially-impassible terrain that you can shuffle around the battlefield.
Everything is about either giving yourself a tool to take advantage of, or denying the enemy the use of theirs. You take units with good armor saves to give yourself the ability to ignore a lot of damage, but the tool of low AP or a power weapon can take that ability away from you, etc.
What tools do you find to be especially useful? Is there something you often take advantage of that others often overlook?