Parallelism Best Practices

What are the best practices with setting parallelism in general? I know that SQL Server defaults to 0 to use all available processors, but in what instance would you want to change this default behavior? I remember reading somewhere (I'll have to look for this article) that for OLTP workloads you should turn off parallelism (set maxdop to 1). I don't think I completely understand **why** you would do this. When would you keep maxdop up to SQL Server (0)? When would you turn off parallelism (1)? When would you explicitly state the maxdop to a particular number of processors? What causes parallelism?