One of the things I've noticed over the last few years, is that it's not just what's being sold, but often what's being given away too: conferences seem especially bad for this "giveaways that you don't want" effect. Life got better when I started looking at it as refusing to take responsibility for (getting rid of) "junk" that someone else didn't want. (Saying you "don't want a conference t-shirt" causes no end of confused looks, at least at IT conferences...)
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Ewen