Step 1: Stay Calm
It helps to convince yourself that you are telling the truth to begin with. You can not be lying if you don't think you're lying. You might be delusional, but that doesn't matter since you have convinced yourself you are telling the truth.
Step 2: Speak in generalities
It's good to offer as few details as possible. Only give details if asked for them. Details can be used to unravel lies.
Step 3: Be prepared
Step 4: Keep eye contact and positive body language
To keep from giving yourself away the first thing you want to do is keep eye contact with the person you are talking to. Unless, you are one of those freaks that never make eye contact with anyone. Then, whatever you do, don't make eye contact.
The best way to make eye contact is to focus on an imaginary spot somewhere to the left of their head around their left ear. This will seem like you are looking straight at them.
Keep a neutral and relaxed posture and don't do anything that would make you seem nervous like shake or scratch your neck. I like to stay relaxed by singing Irish folk songs to myself. The kind of song that goes on and on and on.