Recover your vitality. Certain enemies give up different souls. Long Arms rarely give up vitality, while Reynaldos often cough up life. Get to know who traditionally yields what in order to plan for when to take healing items, etc. Bigger foes often give up more souls.
Bosses drop huge amounts of soul power when slain.
Different colors of souls alight from fallen foes depending on how they were slain. If an enemy is dropped using a critical move, it will give up more souls, and almost always yield vitality. Magic attacks are very powerful and should not be wasted. Face lesser enemies with your regular sword, and use magic on the bigger, tougher foes. Certain magic attacks make you immune to counterattack while you're doing them.