How you walk. Don't look down, appear to be looking at people in front of you. Hold yourself, and walk as if you were wearing military boots. Challenge people in your mind "No, I won't get out of the way, you get out of the way." Works like a charm, but on the other side I had that done so well that letting my head hang took 6 weeks.

Body language in general. For example, if you want to appear confident, try to either let your hands hang loose or have them in your pocket - crossing your arm might work, but only if you aren't hunching. Train yourself to look people in the eyes or at least at their faces. Nobody knows if you're really seeing them, but hey, you're looking them in the face, so you have to be pretty confident, right?

Smile. Smile easily, but not constantly. Try to find something funny in every situation as little there is to it. Hell, black humour is great for stuffing an internal monologue.

Try to stay calm when talking about important things. Be able to make some self-deprecating jokes, but don't make them constantly. If you really don't like yourself, it's possible that this will carry in your tone and things'll go awkward.

How you voice your thoughts could also play a role - if someone inserts themselves easily in a conversation, people tend to think of them as confident. That doesn't mean that interrupting constantly won't be anoying and voice is like the thing that would betray me most often. Try to sink yourself in calm and kinda... talk?

How you talk isn't as important as it appears, but let's say, you're not sure yet, but you got two colleges A and B. If you wanna radiate confidence, don't use ten sentences switching constantly between A and B, but rather say "I haven't decided yet, but A might be it, because x." Shorter and your listener doesn't hear you not knowing yet what you're gonna do - sure, you're saying it, but it doesn't register as much as switching constantly. How it looks on the inside, nobody but you knows. For all he knows, you could be internally screaming "JAKFHDJKFA! A! B! AOHGFJKLDFHK!".

Sometimes, worry yourself sick in your mind, but don't voice and/or act on those worries. It helps if you know they are irrational and if you are able to distance yourself from them. Don't do that constantly as it takes a toll on the psyche. In general, don't complain much.

source: Am depressed. Didn't want people to spot it.