I haven't used the auction in ages.. maybe I should start checking those out again. I remember it being quite competitive but also thrilling.
It depends on the item. If it is very unbuyable and you want LOADS of people to flock to it, set it at a low price (e.g. a 900k item at 100k). HOWEVER, for this, remember to ADVERTISE on the boards. Have a HIGH price increment so that the price will go up fast. Also, set a relatively long time length so that the price will have a longer time to get bumped up.
I've gotten a few - a Grey Paint Brush, Nerkmids, some transmog potions and random codestones xD It's about keeping a lookout for profitable items on the back pages, then stalking the lot up to the front page, then *POUNCE* when it hits #10 on the list, or so (maybe #20, if Neopets is more active in your timezone). ;o