Actually, telling people what you don't believe is negative. Why not tell people what you DO believe instead? Why not stand FOR something instead of stand AGAINST something?
One problem I have always had with the word 'atheist' (or 'atheism') is that negativeness is built into the very word itself, with the first 'a' in either atheist or atheism meaning 'against' (as least as far as I know) rather than 'for'. Compare that with 'humanist', which (although there's no 'for' word in it that you can actually see) still basically means 'for' humans or 'focused' on humans, rather than 'against' humans. So the word 'humanist' or 'humanism' to me seems to radiate much more positive energy than the words 'atheist' or 'atheism' does. Although essentially of course they're both just words for nontheists (see, darn it, another word with a built-in negative, the 'non' in nontheist!)