OK the biggest thing about undercutters is the "I want it NOW!!!" mentality.
often they are weekend warriors (not always) who play all friday night and all sat, and all sunday arvo, they do their farming on friday sat, and then list for sale on sat night, and they want that cash in their pockets before sunday, they see other prices and they undercut because they want a quick sale. they knock 5-20% off to get that sale.
now think for a moment, If you undercut by 1c you are still going to be the cheapest there. you are still going to sell before the others. but you will profit the same amount.
the other point, if your saving for epic mount or whatever, and say your 30% along the way to your goal, undercutting prices so you can get money faster, doesnt get your epic mount faster, if its going to take you 2 months to save, why does it matter if you get 100g from the mats you farmed in 2days instead of 2 hours? why would you settle for 60g in 2 hours because your not going to get the mount for 2 months anyway.
my biggest tip for people that really try to save,
make an alt, a dranei, walk him to exodar and park him at the ah. send him bags and 100g to get him started (for listing fees) from this point on, never sell another thing on your main. every item you have to sell, mail it to your exo alt, (why exo, cause its not overcrowded, it loads faster, and its the shortest mailbox-ah trip. (after 6 months of playing and thousands of trips, the few seconds saved adds up)
send him everything, half stacks, full stacks, gems, greens, herbs, anything and everything you could sell. then at the start or end of the day, log onto the dranei and list what you have to and collect money, keep it on that toon, only transfer money to your main when you need it. if your selling on your main, you tend to check the mailbox, and the ah more often, time you should be out farming or getting more gear or whatever, your waiting on that next sale, when you get that sale pending message, you cant wait to be there to pick up the coin so you can see it in your purse total. BREAK THIS HABIT!!!!!
log onto your alt, list everything then log off, go back to focusing on your main. Dont undercut, if you have to, undercut by a couple silver, or even just a few copper. there are times someone will undercut you, dont sweat it, go farm some more. the most important thing you can do is focus on sending your alt bags full of items to sell. dont worry about what comes back from that toon so much.
I DE and sell mats, i have a set price for my mats, it costs me nothing to relist them if they dont sell, it doesnt matter if i put 20 stacks of greater nether essence up at the right price, it doesnt crash the market, if anything, it steadies the market, it means a price is more likely to stay around that level, if other sellers always see good amounts of something at a set price, they will hover around that price because thats what its worth.
If you worry more about the number of auctions you have rather than the price of each individual auction you will make more in the long run.
I sell pages and pages of crap each day, nothing more satisfying than logging on to see 300-400g waiting in the mailbox.