The Focus thing is really handy for this - provided you only need to deal with 2 mobs.
For example, if you need to dps one target and sheep another target in a chaotic mess of mobs (think a mage during Gothik), you set your sheep target to be your focus ( /focus ) and use the following macro
/cast [target=focus,exists] Polymorph
(from
http://www.wowwiki.com/Most_Used_Macros#Focus)
This macro sheeps your focus target without changing your currently selected mob, so you can keep one mob targetted for dps while periodically just sheeping your focus target.
If you're a tank and need to hold aggro on 2 mobs from a multimob pull (Faerlina/Domo, etc), you can preset one to your focus target, and select the other, and use the same macro (with sunder armor replacing polymorph). That'll get you sunders on both targets early, and you can separate them from the pack a little and then use click targetting afterward.