"If your sword is too short, take one step forward"
- Admiral Marquis Heihachiro Togo

This isn't so much a tactic, as a way of thinking about the game.

40k is a game of circles. With the exception of a couple of tanks without turrets, most units can do anything they want in any direction they want. That means, if you have a unit of guys who can move six inches, their potential move is a circle around their current position, with a six inch radius.

A unit that can shoot 12 inches can hit anything within a circle drawn around that unit with a 12 inch radius.

By extention, we can use circles to see the largest extent than anything on the board can affect anything else. We can also use the same circles to predict our opponent's moves.

For instance, If your opponent has a tank that can move 12 inches, or move 6 inches and fire 24 inches, the maximum range that tank can affect is 30 inches (6 inch move + 24 inch shot). So, if you keep your troops 30 inches away from the tank, they will be safe.

When you consider your own unit's potential area of affect, the more enemies you can catch in overlapping circles, the better you will be doing, as you will then have multiple units that can utilize their force.

Once you have figured out the maximum potential area of affect circle, you can also figure out how much each circle is able to move each turn. If a unit can move six inches a turn, and fire 24 inches (even if moving), then their area of affect is a 30 inch circle, and that circle can move six inches each turn.

Finally, things like terrain will reduce, or even eliminate sections of a circle. Difficult terrain will reduce movement (the average result of rolling 2d6 and taking the higher result is around four and a half) while terrain that blocks line of sight will remove pie-shaped sections from the circle.

Using this approach, you can plan how to set up your units to catch your enemies in your circles, while remaining outside theirs.

In this picture, three units of firewarriors' firing range is shown by the three black circles. You will notice that the circles all overlap, meaning that all three Firewarrior units can shoot at the orks, and can also support each other. The orks movement is shown in the darker red circle, while their maximum charge range is shown in lighter red. The firewarriors are safely outside the orks area of control (at least this turn).