Couldn't find a guide for this so here's mine .
For this you will need:
-MyWarcraftStudio
-a DBC editor
-WoW model viewer
Start by opening WoW model viewer. Load your race/sex and take a look at the (default) top-right panel. Browse facial features until you find yours and put the number in notepad so you don't forget it.
Now open MyWarcraftStudio and load World of Warcraft\Data\enXX\patch-enXX.mpq
Where I write enXX, it'll be enUS or enGB, depending on what client you have installed.
Once you've loaded the .mpq, expand DBFilesClient. Extract CharFacialHairStyles.dbc to where you will find it easily.
Now look in the table below, pick out the race ID that matches your race (before the race swap, of course). Also put that number in the notepad with your facial feature number.
Now open the .dbc file you extracted using any dbc editor.
The left-most row is your race ID (second number you noted). The second row is gender (0 for male, 1 for female). The third row is facial features (first number you noted).
*Update Nov 12, 2008: supposedly you need to change Field 3 instead of Field 6 as of patch 3.x
Haven't tested it myself.
Look for the race/gender/facial feature style combination of your character.
Then change Field 6* (the row after the rows with the big numbers) to either 2, 3 or 4. The image below shows what each number represents.
Then simply create (or edit) an mpq file, then add the edited .dbc file as:
DBFilesClient\CharacterFacialHairStyles.dbc
Then save and close the .mpq file and you're done.
Happy editing, everybody!