How to handle/enforce single instance threading

I have a "worker" process that is running constantly on a dedicated server, sending emails, processing data extracts etc.

I want to have all of these processes running asynchronously, but I only want one instance of each process running at any one time. If a process is already running, I don't want to queue up running it again.

[example, simplified]

while (true)
{
    // SLEEP HERE

        Task task1 = Task.Factory.StartNew(() => DataScheduleWorker.Run());

        Task task2 = Task.Factory.StartNew(() => EmailQueueWorker.Run());
}

Basically, I want this entire process to run endlessly, with each of the tasks running parallel to each other, but only one instance of each task running at any point in time.

How can I achieve this in C# 5? What's the cleanest/best way to implement this?

EDIT

Would something as simple as this suffice, or would this be deemed bad?:

        Task dataScheduleTask = null;

        while (true)
        {
            Thread.Sleep(600);

            // Data schedule worker

            if (dataScheduleTask != null && dataScheduleTask.IsCompleted) dataScheduleTask = null;
            if (dataScheduleTask == null)
            {
                dataScheduleTask = Task.Factory.StartNew(() => DataScheduleWorker.Run());
            }
        }