First impressions are difficult, and you really only have a few seconds to make them. Alexis Ohanian, co-founder of sites Reddit and Hipmunk, suggests that the trick is just giving a damn.
Image: NASA Robonaut.
All three of the companies I helped to start (Reddit, Breadpig, and Hipmunk) have "cute" mascots. I didn't do this because someone recommended I do so; it's just something I did because I gave a damn how my website looked. In fact, Paul Graham long loathed the Reddit alien because he thought people wouldn't take us seriously.
And indeed, a cute mascot is not a requirement for success. What is a requirement for success is that you, as a founder, give a damn about everything -- even something as seemingly insignificant as a mascot. Remember, no one else will care about your website (your mum excluded) unless you do. That means that it's up to you to think long and hard about the kind of brand you're trying to create.
His advice is clearly about making a great first impression with a product, but I think the same notions translate to general interactions as well. You make choices every day about how you present yourself, how you talk to people, and about who you want to be. While you shouldn't stress out over how you appear to others, you should care about the details of who you are and the work that you do. Even if it's a menial task at the office, it's a chance to make an impression. Give a damn, and it'll be a good one.