Great. Thanks for the additional information. I really appreciate it.
So, it sounds to me like she may be having a problem with her anal glands.
All animals have anal glands, and all glands will with that smelly, oily substance. Normally, the glands are emptied when an animal defecates normlly, unfortunately, as you're finding out, some animals have a problem getting them emptied and then they need to be brought in to have them expressed.
There's a great article on them here: http://www.examiner.com/examiner/x-7468-Pet-Health-Examiner~y2009m7d20-Pet-Health-101-What-are-anal-glands-and-what-problems-can-they-cause-for-dogs-and-cats .
As for what you can do to help them...although there is nothing proven to work on every dog, I've seen some success by adding more fiber to the diet (essentially making the feces bigger, so that it pushes harder on the glands on the way out...hopefully expressing them). You could try adding a little canned pumpkin (only real pumpkin, NOT pumpkin pie filling) to her diet a couple times a week and see if that helps.
Otherwise, the only other real option is either having the vet express them for you, or doing it yourself at home. If you're interested in trying it on your own, there's a great video on how to do that here: http://www.expertvillage.com/video/8453_dog-grooming-basics-glands.htm .
Keep in mind that expressing anal glands is a delicate job and using too much pressure can damage the glands. If you are unable to express the glands yourself, I suggest calling your vet and scheduling an appointment because glands that are difficult to express may actually be impacted and forcing the issue can cause serious damage to the area.
I hope this helps