You don't have to be the Brad Pitt of apps, but you should still make the effort to be pleasant looking. At the least, you should be clean. That whole "it's what's inside that counts" thing? It's true, but chemistry matters too, and we're genetically programmed to be attracted to attractive things. If nothing else, wearing your good shirt and combing your hair sends the signal that you care. That you bothered to take a shower before you showed up at our door, says something meaningful. Be clean, be simple, keep the bling to an absolute minimum, and don't forget your mom's advice--"you never get a second chance to make a first impression."
Be fun. Don't be negative. Be the one others want to be around. How do people feel when they're around you? Do they light up a little? Or do they feel inexplicably darker and less energetic when they spend time with you... Hint: make a list of the apps, products, APIs, frameworks, etc. that make YOU happy. The ones that make you think, "this is awesome." Or better yet, the ones where you never think about them at all... because you're too busy being awesome doing the thing that led you to that tool in the first place. Focus your energy on putting yourself on someone else's "makes me happy" list.