If someone finds out that you are lying, don't make excuses. Come clean, although tell them only what they already suspect. For instance, say Sally decides to make up a boyfriend and names this made-up boyfriend Joe. She talks about Joe to her friends, Emily and Kate. Emily takes Sally aside and asks her if she and Joe are REALLY dating. Sally would then, rather than telling her EVERYTHING, only tell her what she already suspected, so she would say something like, "Joe and I dated for a really long time, but I broke up with him. I'm just saying that we still go out because I don't want to date anyone else right now, so I want people to think I'm taken- please don't tell anyone so they don't try to ask me out," rather than, "No. I made up Joe." This way, Kate is still under the impression that Joe is alive, and dating Sally, and Emily still doesn't know that Joe never existed in the first place.