So you've got yourself a Death Knight you want to use to farm some gold, but aren't sure which spec is optimal? Well, to be honest you can do fairly well with any DPS spec, but one spec in particular is ideal for DK gold farming: Blood. Blood allows for decent health return, eliminating the need for downtime, and is handy for it's ability to kill two mobs simultaneously with little problem.
We'll be using a typical DPS spec: http://www.wowhead.com/?talent#jfVMVh0Icfkf0stZfMbhxc:AILVm
Now that you have the spec, time to discuss the fun part: killing mobs. Blood is a pretty straightforward spec, and you typically won't make it very far at all into your rotation. Whenever possible, grab a few mobs at a time; they won't be able to hurt you enough to matter, and you'll kill that much quicker.
Rotation: IT, PS, HS, HS or Pestilence (depending on number of mobs), DS, DC, HSx4, Pestilence to refresh diseases, DS to get more death runes
DK gold farming is actually pretty simple! Heart Strike and your diseases will be hitting multiple targets, so again, pull as many enemies as you feel is convenient; just don't waste time trying to gather too many enemies, as it is often faster to dispatch a smaller number of mobs, mount up, and quickly move to the next set.
Other than that, just use your head. Use your cooldowns intelligently; don't blow Dancing Rune Weapon on a half-dead target; heck, if you're killing enemies fast enough I'd advise not using it unless you have a few healthy targets ready to burn. Same goes for your Ghoul. Be smart about using your cooldowns, and you'll greatly increase your kill speed overall.
I hope this little guide to DK gold farming has helped you out some. Anyone looking to further optimize your gold earning abilities should check out my Gold Guide Review, where I compare various popular gold guides, and find one that sounds good.