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Well, I found this on about.com; hope it'll help:
Spanish has two common adverbs that mean "never" - and you can almost always use them interchangeably.
The more common of the two is nunca:
Nunca te olvidaré. I will never forget you.
Brittany y Pablo nunca fueron amigos. Brittany and Pablo were never friends.
El presidente no ha hablado nunca a favor de imponer sanciones. The president never has spoken in favor of imposing sanctions.
Nunca quiero que llegue ese día. I never want that day to come.
Less used, and perhaps a bit stronger than nunca, is jamás. In all of the above examples, jamás could have been used, and in the following examples nunca could be used instead:
Es el mejor libro jamás escrito. It's the best book never written.
Jamás pienso en la muerte. I never think about death.
Jamás imaginé que llegaría este día. I never imagined this day would come.
Quiero dormirme y no despertarme jamás. I want to fall asleep and never wake up.
One of the very few times you cannot substitute jamás for nunca is in the phrases más que nunca and menos que nunca, which mean "more than ever" or "less than ever": Mi hermano gasta más que nunca. My brother is spending more than ever.
Note that when nunca or jamás follows the verb that it modifies, a double negative sentence construction must be used:
No he visto a nadie jamás tan malo. I have never seen anyone so bad.
No discutas nunca con un imbécil, te hará descender a su nivel. Never discuss anything with an idiot; he will bring you down to his level.
Also, nunca and jamás can be used together to reinforce their meanings, much like "never, never" or "never ever" in English:
Nunca jamás vayamos a aceptar una dictadura militar. Never, ever are we going to accept a military dictatorship.
Nunca jamás hablé con nadie de esto. Never, no, never have I spoken with anybody about this.
