Okay, simple straightforward advice. Do not lie. Don't lie to your girlfriend/boyfriend, don't lie to your boss, don't lie to your friends and don't lie to your dog (especially not to your dog). It's not worth it. Lies tend to escalate, cause stress, cause pain, and can also cause embarassement.
An adendum to this is that sometimes little white lies are okay. Sometimes. However you should take care to use of them wisely and at the right time because you don't get an unlimited supply of them. Evenutally the person you're passing them off on is going to catch on.
my mum: Katie, is that what you wore to work today?
Katie: ummmmmm yes.
mum: don't you think it's a little unprofessional? I think people might get the wrong impression about you.
Katie: what, that I'm a slut?