As a former warlock, I feel you pain there.
Like most features of WoW it takes a long time to get a UI built around your preference, and preferences vary dramatically from player to player (I've seen some UI's that make my toes curl). My warlock's skills consisted of almost exclusively macros to help with targetting.
Here's a small list of targetting assistance I've used to avoid excessive tab usage.
Gladius is a terrific add-on that's almost vital for arena. Along with the great customizable UI to keep track of players, it also allows you the ability to build in specific targetting macros.
This will target the first player on your gladius frame. By setting up keybinds for my most common arena setups (2's and 3's) I could quickly dot both targets without ever having to tab or click anyone.
Ok, you've probably noticed this by now. But the theme here is macros. As a warlock a lot of the 'one time' spells you use lend themselves very well to mouse-over macros. Things like banish and fear are natural fits.
/cast [target=mouseover,exists] Lifebloom
This is a must as a warlock, the ability to fight two people (or monsters at once) is vital. In addition to the ability to dot both, you can also enable their casting bars for that well timed silence.
While this list is by no means exhaustive, it should help you get started and building out your own customization. Dot timer's can be invaluable as a warlock, as it will allow you the ability to keep track of former targets better as well. There are some addons that help with targetting such as gladius, but most players make extensive use of macros and focus targets. You can find Azael's macros here (arguably the best pvp warlock) which should give you an idea of how to set up some of the more gritty macros.
Hope this helps.