I had quite some exchanges allready - others did not happen in the first place. Mostly the exchanges didn't work well for me - I think for those reasons:
- One language partner wanted to chat - the other wanted to learn in a more seriously way. Funny: Sometimes I was the one who wanted to chat more and in other exchanges I was the other part.
- The knowledge (of the language he was learning) of one partner was above the one of the other partner and we spoke much more in one language.
- We both didn't speak the new language well and ended up speaking English.
- Language partners didn't stay in the language, which was on at that moment - happened all the time !!!
- Language partner corrected every mistake (far above the level of their partner) but then refused (or could not) explain.
- Tendency to talk about the same topics all the time and you do not really go into new vocabulary - a problem even when you get along with a partner. And then it gets boring.... but you do not find new topics.
- Language partners tend to not value the time the same way as they would value the time of a (paid) tutor.
Now to answer your question:
- I always try to find a partner with the same German level as I have in my target language.
- I try to find out about the goals of the other person before we start the first session. You will see that many do not have a goal!
- Agree about the form (chat or learning) and how much correction each of you wish. Try to stay within the limits of your partner.
- Agree about the length of the session, to stay in one language for 50% of the time and then to switch.
I used to do quite a few exchanges on here before I was a teacher. For the video I tried to do 50/50 exchanges on Skype using video or audio. I prefer video but if the other person likes audio, that works too. I usually try to start in English to be polite, although when you're starting you might do like five minutes in each language so that they don't practice their English and then take off if they're selfish. I found some really good partners.
The ones that didn't work were for one of two reasons. Sometimes the person just has different objectives than you and personalities don't mix. The other one can be that the person wants to practice your language a lot more than they want to help with theirs. It doesn't happen a ton but it does happen occasionally.
I just used trial and error with messaging people, you can start with people who have a promising sounding message about exchanges in their profile, or you can just start contacting English learners. Our native language is one that a lot of people need so that helps the search. You should be able to find some really nice people for exchanges.
@Javier: I read your notebook entry and agree on many points. But even when a tandem partner does not need to be a friend, I find it much easier to talk to a person close to my age, education, interests.... which more likely turn out to become a friend.
I also had a positive experience (should have mentioned that before) where someone who I helped even outside the tandem (corrected his CV and cover letter - which is part of my occupation) later was concerned that we should speak more Spanish, as I plan to take an exam.
One other thing: I now often prepare a bit of "lesson" for my language partner as well as for me. What I found interesting is to exchange a picture and then talk about it (what do you see, what do the person do, what might they think/feel/wish.....). I also work with a magazine called "Ecos", which is in easy Spanish (to discuss or read an article) and even buy now and then the German equivalent called "Deutsch Perfekt" for the same reason.
Last week I wrote a notebook about this issue, especifically about meetings on Skype. Take a look, if you want, maybe could give you some ideas.
(I've voted up Brit's comment, I'm totally agree with her)