i tend to ask the hostess what is needed - if i get a vague "i dunno" answer, i'm the first to buy something in a packet that i don't care if it gets opened or not - so much better to have them stay fresh than lots of wasted food! if i'm asked to take home something i've taken cos there are left overs, i'll usually organise with others in a similar situation to "share" stuff so we take a mixed plate each - and then feed it to DH if he hasn't been there... if we take freshly baked food, he's usuallly baked it anyway...

if i'm having something here, and people offer to bring something, i tend to ask what so that i can make sure we dont triple up on one thing, have nothing of another... it works for us pretty well