I realise there are already many guides posting on how to do this, in-fact I learned from a guide here. But I was told this guide I made for my guildies is good so I'll go ahead and post it.
First things first, you will need a Hex Editor.
I use: RapidShare: 1-Click Webhosting
Be sure your not in wow and open up XVI32.
Once it is open, you will get a big grid, maximise the program. Click on File-->Open (or just the quick button) and you should get to a screen similar to this:
Navigate to your way to your Wow Folder-->Cache--->EnUS--->Itemcache.wdb and open it. I suggest first making a backup copy of your Itemcache.wdb in-case you mess something up.
Don't freak out, there is lots on the screen and again, don't freak out...this is the part most people say no thx.
Now that your in your Itemcache, go to Search-->Find(or ctrl+f). Be sure all your options are the same as mine and type in the Item you are wanting to change, be sure to have everything exact, including punctuation.
Hit ok and it should move you farther down into the jungle of numbers.
Now on the Right Column navigate yourself to the very last letter of the last time the Item is typed and select it by left clicking once. You will notice the left Column will react, don't worry this is supposed to happen. You should now be at:
Now, in the LEFT COLUMN Move to the right two boxes so that the box you have selected is the one directly after the 00. The two boxes (4 digit/letter total) contain your items Hex Code.
You are going to be changing the Hex code to the item you want to see, go to WoW Hex Code Search and type in the name of the item you want to change to.
Take the new hexadecimal code you now have (in my case 8 e 7 7) and go back to XVI32 and simply type the new code, the box will auto change so dont worry about hitting space etc...
All that is left to do now is Files--->Save (or ctrl+s), close XVI32 and open wow to see your new fancy items.
Should you have done everything correct you should now have the item when you log in (not on the character screen)
Although this may seem complicated, its really not and you aren't adding any files to the data folders or using a custom exe or anything (for those paranoids). Once one gets good at this, one can change their entire set of armor in just 5 minutes or so. Anyways, ask questions if you get confused anywhere along the way. Also, for multiple items obviously just rinse and repeat.