Summarize:

Perhaps the single best way to avoid attracting attention to yourself is to do what everyone around you is doing. There is safety in numbers — if your behavior is indistinguishable from that of the people next to you, it's unlikely that you'll be singled out for any sort of special attention, especially if you're in a large group of people. If you're looking to blend into the crowd, start by examining the "crowd" around you. Ask yourself: What are the people around me doing right now? Are they talking to one another? If so, how friendly are they? How do they seem to be acting? The answers to these questions should help you match your behavior to those around you. Obviously, this step is very context-dependent. While sitting quietly in the corner and reading a magazine may make you virtually unnoticeable in a crowded waiting room, it can get you noticed at a wild house party, so pay attention to your surroundings if you're looking to go undetected. Even if you perfectly match your behavior and attitude to that of the people around you, you can still easily be noticed if you look a way that's not expected in whatever situation you're in. Some fashion choices, especially tattoos, are long-term, so there's not much you can do other than conceal them. However, the clothes you wear and (to a lesser extent) the way you wear your hair are under your control, so keep these things plain and ordinary if you're looking to go unnoticed.  Here's a sample "casual" look that shouldn't attract any attention in non-formal settings. Best of all, it's mostly unisex:  Jeans T-shirt Plain hoodie Athletic shoes/sneakers Clean-cut haircut (for men), straight hair or in a bun (for women) Light makeup, minimal jewelry (for women)   Note that ideas for a wide variety of other cheap, unassuming outfits are available online on budget fashion blogs or simply via a search engine query. One sure-fire way to get noticed is to speak up when it's not necessary, so, if you're trying to blend into the crowd, you may want to try to "clam up" when your opinion isn't required. For interactions you do have to have with others, keep your responses polite and friendly, but brief and to-the-point. The more time you spend expounding on any topic, especially personal ones, the less anonymous you become. Again, this advice is entirely context-dependent. While keeping completely silent may help you on the bus, it will probably call unnecessary attention to you if you do it when you're called to answer a question in class. "Keeping your opinions to yourself" also means knowing when saying nothing at all is more attention-grabbing than an ordinary response. Eye contact is a powerful social tool — it's a way to forge an instant connection with someone else without exchanging a single word, though some research in recent years has shown that it's not necessarily useful for getting people to like you or agree with you. Regardless, if you're interested in remaining completely anonymous, you'll want to avoid other peoples' eye contact except in social situations that demand it. This means that when you're walking or driving outside the house, you should avoid meeting random peoples' gazes and that when you're in a situation where someone might talk to you, you'll want to avoid eye contact until conversation has started.  Some people who are naturally shy or socially awkward can have a difficult time maintaining eye contact in social situations. Because being unable to maintain any sort of eye contact can call as much attention to you as using too much eye contact, you may want to practice with a willing friend or even the TV or mirror if this is the case with you. Luckily, medical research suggests that practice can be effective for improving eye contact skills. This one is a no brainer — if you're trying to be a complete wallflower, don't approach other people and try to start conversations. If you're in a social situation and someone comes to you to start a conversation, you'll obviously want to respond with politeness and honesty, but don't start conversations with strangers you don't have to. Instead, try hanging back and talking with one of your friends or simply taking in your surroundings on your own. As noted in the advice above, you'll go more unnoticed by strangers if you don't come into contact with them at all, so try to spend some of your free time doing things that don't require other people at all (or only require a few close friends). There are hundreds of fun, exciting things to do that you don't need anyone else for which can allow you to have a great time and grow as a person while ensuring you keep a low profile. Below are just a few ideas for fun solitary hobbies:   Learning to play an instrument or composing your own music Reading Working out (e.g. running, swimming, biking, lifting weights, etc.) Studying a new skill Exploring the wilderness/Geocaching (Note: always tell someone where you're going before venturing into the wilderness on your own) Writing (e.g. short stories, blog posts, writing for user-supported websites, etc.) This is the first commandment of wallflowers and people who don't want any unneeded attention. Avoid publicly going against the group you're with. Don't act ways that the people around you aren't acting. Don't dress, behave or speak in ways that aren't considered "ordinary". Don't question society or your role in it. If you do harbor any sorts of disagreements that challenge the status-quo, keep them to yourself to avoid the extra attention!  If it's not obvious, a life that's dedicated only to staying anonymous can be seriously lacking in opportunities for self-expression. Try to keep things in perspective — the attention you get from challenging the status quo can be scary, but it's better than constantly being afraid of the consequences.
Match your behavior to others'. Look utterly ordinary. Keep your opinions to yourself. Don't look people in the eyes. Don't approach other people. Enjoy solitary or anonymous pursuits. Don't break the mold.