1. Refer to your fake boyfriend by name. Friends are generally very difficult to fake a relationship around, particularly if you were planning on making it a long-term illusion. Although you'll always have to deflect the offer when they ask to spend time with him, you should put detail to good use. Refer to him by name, and treat it as if you're talking about a real human being.
The choice of name is important. Avoid outlandish or foreign-sounding names.
You should avoid giving out a surname as well. Your friends may be curious and try to look him up on social media if you do.
2. Think up a personality for your beau. Any illusion has a better chance of working if you put extra thought into it. Because friends are going to want to know more about your boyfriend, you should prepare a believable personality for him. What does he do for work? How did the two of you meet? What are his personality quirks? If you're going to act like you have a boyfriend around your friends, you'll need to dig beneath the surface and concoct a well-rounded story.
Believability includes not making your boyfriend seem too perfect. Although you don't need to list out his flaws along with the good things, you should limit the good things so you don't seem like you're exaggerating. Everyone has a complaint about their significant other from time to time.
3. Avoid trying to impress anyone too much. If you're already dating someone faithfully, you likely won't be going out of your way to impress other people as much, especially when they're the opposite sex. Even if you're regularly extroverted, you shouldn't go out of your way as much to spark conversation with new people.
4. Feel secure and happy in yourself. Many people place a lot of their self-worth on whether or not they're in a relationship. If you want to make it seem like you're with someone, you should do your best to exhibit this confidence in yourself. One way or another, you shouldn't let your relationship status affect how you feel on a normal basis.