With an understanding of the core mechanics behind the Death Knight class and its reliance upon diseases, the next step to effective use of your death knight is to create your personal optimal combat rotation.

There are four key things to take into account when creating your combat rotation; death runes, diseases, runic power and time, each of which has the following effect.

Death Runes – By including death runes in your rotation you can create attack patterns that deviate from the normal rune set, how this will be applied varies depending on your talent choices.

Diseases – Most specs will want both diseases to be active at all times but Frost can get away with only utilising Frost Fever; the talent Epidemic also extends your disease duration allowing you to utilise two sets of runes before having to refresh your diseases causing a longer rotation.

Runic Power – It is important to ensure you never waste your runic power by letting it sit unused at maximum; you should plan for how much runic power will be generated in your rotations and look for ways to use it when you are waiting for runes to recharge

Time – Ultimately you only have a set amount of time to utilise your attacks before you need to refresh your diseases, you runes recharge and your rotation ends; using only single rune abilities will cause you to go beyond your disease duration by the time you account for the time it takes to use up your runic power.

By taking these factors into account, the best rotations will always be the ones that use up all of your runic power and every rune you have, before you need to refresh your diseases.

To give you an example of how to plan out your rotations I will show you the thought process behind my personal Frost spec rotation.

Icy Touch -> Blood Strike -> Obliterate -> Obliterate –> Frost Strike -> Frost Strike.

This rotation is designed for single target damage and uses Blood Tap on the first rotation to swap a blood rune for a death rune in order to fit Obliterate twice into the rotation. With the talent Blood of the North, the blood strike in the rotation will generate a death rune to fuel the following rotation.

In total the rotation consists of 6 attacks which will take a total of 9 seconds to pull off, well within the disease duration and fitting in nicely with the regeneration time of your runes.
Untalented the rotation would generate only 50 runic power, which is not enough for the two frost strikes but the talent Chill of the Grave increases this to 65. By using the Glyph of Frost Strike to reduce its cost to 32 runic power; 65 runic power is enough for both strikes.

If I wanted to replace one of the Obliterates for an additional Blood Strike and a Plague Strike, the rotation would become sub optimal since it would generate more runic power than it utilised as well as take longer to execute than it would for your runes to refresh.

By trying to take all the factors above into account you can create a large variety of combat rotations for different situations such as PvP or AoE. Finding the right rotations for you is half of the challenge of the Death Knight class and is what you build your talent and glyph choices around.