Take a look at Mosg2's DK Manifesto.
There are two versions.

That should get you a decent starting point.

Your choices are: Blood, Frost or Unholy.
Each has a preferred Runic Power dump.
- Blood = Rune Strike
- Frost = Frost Strike
- Unholy = Death Coil

Beyond that, you have two runes of each type.
And they refresh 5 seconds after they're used.
So as long as you have no more than one of each type of rune (per 5 seconds) or more than two of each type (per 10 seconds) the runes will replenish as you use them.
You would ideally like to use the runes a little faster then they regenerate, so the Runic Power dump can fire off.

What spec are you looking at?