I am sure this is well known to most players but I thought I'd write it up just in case somebody finds it useful.
While soloing it is common to come across groups of mobs that are close enough together that hitting one will arouse the whole group. Most classes can handle two equal mobs at a time but three or more can get hairy so it is useful to be able to pull one mob away from a group to tackle on its own. As long as you have space to run to this can usually be done with careful pulling.
The basic principle to remember is that a mob you have damaged will chase you for longer than a mob you haven't. First make sure you have a safe path to run away. Then pick the mob you want to bring down first and damage them with a ranged attack. Then turn and run down your safe path. Glance over your shoulder occasionally to see if any of your pursuers have given up. Those you haven't attacked will generally retreat before the mob you have damaged. Be careful not to run too far or even that mob will reset.
It is generally best to pull melee mobs first when doing this because they are sure to chase you but be careful that you get out of range of any ranged pursuers before standing to fight.