I subscribe to the "give before you ask" hint:
- a good quality correction of a text shows that you are proficient in your own native language (something that you cannot assume for granted);
- if you are helpful to strangers, you'll be as helpful as a language partner, not like some leeches who try to spend all the time speaking THEIR target language.
Another hint: if you want to receive language exchange offers, open your privacy settings, so that other italki members can send you private messages.
I know that you'll be flooded by spam messages, the problem is so bad that even Miriam had to "rise her shields", but once in a while you'll get a real, serious proposal.