Lying is easy. Ever had someone ask you "how are you ?" and you answered "fine, thanks" though it wasn't the case ?
Now being a good liar is more than lying easily. It's to be able to lie without being detected AND to maintain this lie over time.
For the detection phase, that's basic information handling, so you have three things that could mess up with it :
- no information
- contradictory information
- too much information at the same time
In this regard, when you know what the other person is looking for, you have an definite advantage because you can starve, confuse or overload the other person's "detectors". Since you know what those detectors are, they are easy to handle.
For the consistency part, that's pretty much a brain thing, body language won't help you to remember what you told to whom, to invent something plausible in a tenth of a second (or find something to delay), to find other lies to add on top of it, to mimic your usual vocabulary/chattiness...