All it takes is hot water!
I got this tip from an antiques dealer in Gettysburg, PA. They specialize in Civil War era firearms and told me they simply give the entire disassembled firearm a bath in boiling water. They said they leave the components, including the wood stock, in the water only for a minute- just long enough for the cosmoline to liquify and run off.
Here's my application: I recently purchased a Mosin-Nagant 91/30 and it was covered in cosmoline as you'd expect. I wiped the bulk of the cosmoline off the rifle with a rag before disassembling it. The small parts were washed off in the kitchen sink and larger parts in the bath tub. I boiled several pots of water and poured them over the parts until they were clean. I used a pan to prop up the breach end of the barrel and poured hot water down the bore. Using tongs and mitts to handle the hot metal parts, I turned each and repeated the process. Personally, I couldn't bring myself to risk messing up the wood with the hot water, so I mixed some dish soap and warm water and wiped the stock down until it felt clean and dry to the touch.
After the parts cooled, I went over everything with Break Free CLP to clean and lubricate as well as prevent any rust. Cleaning the bore only took 5 or 6 passes before I was getting a clean patch. The entire process took just under an hour.
It's a simple process that takes a lot less time and effort than some others I've heard about.