This will undoubtedly lead to the question of getting back together, and will sabotage your chance of establishing a platonic friendship. While maintaining a friendship with anyone, ex or not, requires occasional contact via phone or computer, don't start texting or calling your ex on a daily basis. This will inevitably create a false hope of getting back together. If either you or your ex is getting too attached and old feelings start to arise, then don't force the friendship. This is probably a sign that the breakup is too fresh, and that you need to spend more time apart.

Summary:
Don't have sex with your ex. Don't fall back into old patterns. Know when you back off.