Never show up empty handed. And always give your hosts something they can use after you've left. That's some good motherly advice.
But it's not just about bringing something, it's about bringing something special, that matters. Every hostess gift I give is measured against the box of 24 beeswax candles a friend brought to my house one evening when he arrived for dinner. Now that was special.
We try to channel that friend whenever we visit our friends and arrive with something special that our host can use and most importantly, will love.