In Dragon Age Origins, why does my Mana keep draining during battle?

Sustained spells continuously use mana for as long as they are active. To counter this you need to equip your mage with an item or items that have mana regeneration for exaample the guy selling stuff in the tower of magi sells an awesome belt that has something like +4 mana regeneration during combat (sorry don't remember the name of it but its the one that costs like 100 sovereigns). He also has an awesome staff that has mana regeneration (also around 100 sovereigns) and at the mage shop in Denerim there are some tevinter robes that have mana regeneration and some hats too. So just equip your mage with as many of these mana regeration items as you want and that should help keep your mana up while still using these sustained spells.