You need the command materia W-Item equipped in battle. Go to W-Item and select any item. Confirm that item then pick it again until you have to choose who you want to use it on but don't confirm it. Press X to cancel. Then pick the item again until you have to choose again and cancel. You'll know that the item will go up by 1 each time you do this.

You have to have at least 2 of the same item to do this and you can't exceed over 99 when using the glitch. You can't use the glitch with weapons, armor, accessories, etc.

This glitch is neccessary if you're the type of person who wants to get everything maxed out.