How do I stop my guinea pig from biting?

Question
Hi Andy,
She is biting when I pick her up and when I'm petting her on my lap.  Am I doing something wrong to aggravate her?  I've heard that you are supposed to pick guinea pigs up often so that they get used to being handled.  Thank you for your help with this.-------------------------
Followup To
Question -
Hi Andy,
We've had our guinea pig for almost 3 months now and she continues to bite us even though we're gentle with her.  My boyfriend and I are the only ones that handle her, so I know that she is not being mistreated.  Is there anything that I can do to stop her from biting?  
Answer -
Hi there.

Guinea pigs don't bite because they are agressive. In fact, they usually nip you when they need to go to the toilet.

Can I ask, how frequent is this biting? Is it whenever she is picked up or does she bite when she is on your knee?

Andy

Answer
Hi there again

Perhaps your pig is of a nervous disposition. Also, when you are petting her, she will tell you if you are doing something wrong because she will throw her head up in the air.

Also, it depends on how she bites you. Does she draw blood or just nip? Sometimes pigs put their mouth round your finger etc as a sign of affection.

Andy