So, you lied to your girlfriend. Bad move. How do you earn her trust back after lying?

[edit] Steps

Be honest about lying. Explain your reasons for lying, if you had any. This is essential.
Apologize. This is the most important step. Be sincere and specific in your apology. Acknowledge what you did wrong.
Make a promise to yourself that you won't lie to her again - and don't! The only way you are going to ever get her trust back is to be honest. Remember - once burned, twice shy. This only depends on what happened.
Don't expect her to jump back to you. Don't be quick to say "I will never do it again" to her. After hurting her once, there's little chance that she will be convinced. You need to tell her that you don't expect her to trust you again, which goes hand-in-hand with acknowledging what you did wrong.
Gauge her reaction. Is she still giving you the cold shoulder? That's okay. She's been hurt. Eventually, she will warm back up to you.
Give her time. You've seriously hurt her feelings, and, depending on the severity of the lie, possibly damaged her trust beyond repair. Don't apologize and expect everything to go back to normal. Understand that she needs time to heal and eventually forgive you.