Mouse and Keyboard users can just press the T key. PS3 / gamepad users will need a macro.

I use the following macro to assist: /assist <t>

In example, when healing or DD I will target the tank in the party quickly using up/down on the dpad. Then I'll press the assist macro which is slotted on my main battle hotbars to target the tanks target. This make it much less likely to pull the monster to yourself as the tank will have high enmity (hate) on their target. If I was to attack another monster which the tank isn't fighting it's likely it will focus attacks on me. This make it harder for the tank / healer to keep you alive.

I do think assist is extremely useful and wish the developers had made it an action with it's own icon so more people would use it.