This site has good tips about tandem partners.
http://www.slf.rub.de/etandem/ethilfen-en.html
http://www.slf.rub.de/email/help/helpen ... #questions
http://www.slf.rub.de/tandem/inh01-eng.html
I would agree with untmdsprt that you need someone of a similar ability to yourself. You need to avoid defaulting the the strongest language in common. This is fine for communication but no use for learning.
It can be a bit artificial and weird at first. Meeting a stranger for conversation. What shall we talk about is always a bit of an issue.
Explaining one's native language is actually much harder than you might think too.
Is good to be regular in your meetings. Once a week at a particular time. You both need to show respect and consideration for each other.
For me, I've had email tandemn exchanges just disappear suddenly or been unable to continue them. Skype is better in respect that it's easier on your time than email and has the added benefit of real-time and speech.
I've had a Skype partner for about 2.5 years now and it went beyond language exchange to being a friendship where we both help each other with language.
I found that I really only had time for one tandem partner.
Another thing to consider is a language club. In London there is a Japanese meetup. It's run by a friend of mine who has very good things to say about it, however I can never make it to their meetings.
http://japanese.meetup.com/34/
I'm sure there are similar meetings in other cities.