This is part of an old saying, "Keep your friends close, but your enemies closer". I have been following this old saying while doing my glyph business.
If you are going to be running a business in WoW, you should know who your biggest competitors are in any particular market. You should also know their habits.
One thing that I have started doing is putting my competition on my friends list. Not because I like them, but because I can see when they are on.
Most of your competitors will most likely be bank alts parked in one city. If they are on, they are most likely putting up auctions. If you see them on, check you auctions to see if they are undercutting you.
You can also make them play into your hands if you know how they react to situations. One of the sellers on my server puts up one glyph at a time and gets pissed if you undercut him by one silver. He will then take his down and try to sell his one glyph for 3-5 gold more. Someone buys his, and then yours start to get bought up. He loses profit because of being vindictive.
So, keep your enemies very close and enjoy the profits.