Cross promotion is one of an Indie's greatest tools.
On paper, it gives an artist access to potential fans they might not have reached, complete with a ready made endorsement from someone who's work said potential fan already (presumably) likes. If used with a little bit of forethought, and a splash of common sense, cross promotion can be efficient, far reaching, and most importantly free. Here are a few things to think about.
1. Don't Keep Score
Sending out CAPS LOCKed emails scolding someone for not commenting on our latest status update, re-tweeting you, or only posting three of your links when you shared four of theirs just isn't helpful. Take things as they come and keep it moving.
2. Say "Thank You"
This is probably the easiest thing to do, but the hardest one to remember. Time zones, crowded news feeds, and busy Real Life make it difficult to catch everything nice that someone says about you. On the occasions that you can, use the two magic words that your Mother taught. They still make a difference.
3. Remember Your Non-Musical Friends
Bloggers, photographers, filmmakers, painters and poets need love too. Most other artistic mediums go hand in hand with music anyway and Indies of those other ilks can help spread your sounds to a completely different type of listener. Which is sort of the whole point.
4. Use All The Forums Available To You
On the internet, the possibilities are endless. Resist the urge to look at everything through Facebook colored glasses. Reach as far and wide as time, net savvy, and motivation will allow.
Indie artist solidarity is a wonderful thing. Use it.