Most importantly, in advance, you must force yourself to believe your lie is truth.This will make you naturally act as if you were telling the truth. The trick is convincing your sub-concious you're telling the truth.
Practice lying in front a mirror or video camera.
Don't lie too much or you might be caught. Don't be elaborate or over-embellish.
You can avoid the entire issue of having to defend your filthy, filthy lie by simply sliding the lie into a casual conversation. It's better to lie to the person in advance than have him question you later on the same topic. That way, the victim's mind has hours, possibly days to fill in the blanks, and if he/she does come back to question the story, you'll have a much easier time as your victim starts on the defensive. If the victim discovers your misdeed before you explain yourself, they'll have time to deduce what has happened with a certain degree of certainity before they ever even question you.
Know your lie before you say it; don't make it up off the top of your head. If you have a hard time lying, try bending the truth.
Example: Q: Do you know where John Smith is?
A: No, I don't know where he is. (Although you might know he's at his house, you don't know which room he's in.)
If you know you're going to have to lie, think of some specific true thing (place, person, event, story) that your lie will fit into and use those details if you are questioned. This gives you a bank of specific details to draw on so you don't have to keep making things up as you go along. The more things you have to lie about to support your original lie, the more likely you are to be tripped up.
Remember to keep a basic idea of the lie, so if questioned deeper, you can stick to the same concept.
When speaking the lie, look the person in the eyes. People lower their guard and assume honesty when someone is able to look them straight in the eyes. (This takes practice.)
Remember the original lie and stick to it. Telling a lie is a lot like story telling; take the original concept and expand on it as necessary. Always keep the lie believable and simple. Don't add extra detail. It only makes it easier for someone to catch you lying.
When you lie, tell them the lie and one fact because if you tell too much, they will suspect you of lying.
Don't tap your feet because this shows you are nervous and you may get caught.
The best practice is watching how you tell the truth, and imitating that.
Plan ahead of time.
You must have a good reputation.
Don't blink a lot.
Remember, it's not a lie if you believe it.
If you do tap your feet or fidget during your lies, make sure to do it a little before and after your lie to make it seem less suspicious.
Practice facial expressions in the mirror. Try making your eyes go big and letting your mouth hang open a little for an innocent/shocked look. Also, practice looking like you're holding back tears. When you smile, show your teeth a little and crinkle up your eyes and cheeks. This is a 'sincere' smile, an ear-to-ear one.
If you're lying to a peer, be assertive or aggressive. Act offended that they would even think that you'd lie to them. Being intimidating is helpful. This won't work if you're generally not assertive in a relationship.
Lie about something small or play a trick on someone. But do it very unconvincingly. Use facial cues like sucking in your cheeks or 'trying' to keep from laughing. The person who witnesses this faux attempt at lying will now think you are crap at lying and not be suspect in the future.
If you sense that someone else suspects you of lying, admit to something small or untrue. They will take the bait and think that that is all you were lying about.
It is always best to remember a lie for years, or keep refering to it in conversations, as if it relly is true.