Addons.
Gladius is a very useful PvP tool that provides you with unit frames for your opponents, allowing you to perform actions to the unit frames, themselves. Mine is set up so that right-clicking a unit frame makes that player my focus target. I also have other clicks set up, such as Shift + right-click, which causes me to use Polymorph on the clicked target without ever having to have them targeted or focused.
More recently, I've dumped Gladius and just used Clique, which is a good all-purpose tool for handling clicks on unit frames (raid frames, arena frames, boss frames, etc.). You can set up clicks to do the same thing as Gladius, but you have to specify that the click is meant for a non-friendly target.
In large-scale PvP, Battleground Targets is similar to Gladius, but provides less detail per enemy (since it can show up to 40 targets), reducing each target down to a customizable bar and icon(s). By default, left-clicking an enemy will make them your target and right-clicking an enemy will make them your focus target.
These addons are very commonly used throughout the PvP community and help you to easily make informed decisions and quickly target the correct enemy.
Alternatively, if you don't want to use addons, you can quickly set your current target to your focus target with the macro command, "/focus." I have a macro that I use for this purpose:
/focus [nomod]
/cleartarget [mod]
When using this macro, you can set your current target to your focus, clear your current target (by holding a modifier, such as Shift), and clear your focus (provided that you don't currently have a target) since /focusing a non-existent target clears your focus.
You can also use macros that target specific players. For example, in a 3v3 Arena match, you can target each opponent using arena1, arena2, and arena3 like so:
/tar arena1
If you have space in your keybinds (eg. the Num1, Num2, and Num3 keys), then you can change targets quickly by just pressing a button.
