I just learned this by reading up on it a bit today. Prestige is xp given to you in your base xp for dungeoneering. It is kind of tricky to understand at first but this is what I grasped off it.
Prestige is an xp given to you per floor you are doing. If you have done that floor already you will not get prestige xp the next time you do that floor, so you get his xp only once. The more floors you do the higher the xp will be given per floor done (not per higher floor number, its total floors.) It was set up this way so people didn’t grind the same floor over and over for XP.
Method of getting max prestige xp. Start at floor one and work your way through the floors. You will get prestige xp per floor done. Upon reaching the highest numbered floor you can do (example lvl 6) go back out and using your ring reset it(you won’t lose floors unlocked or complexity.) Upon reseting go to floor one and work your way down again. This way you will get max xp from prestige and your base xp. This means more xp for ya.
I hope this helps some. And if you have any questions i’ll try to answer them.
RS Dungeoneering prestige link and faq http://www.runescape.com/kbase/guid/dungeoneering_faq