Here's a trick I use when packing wheel bearings - once you've throughly cleaned & dried the bearing, put it in a "zip-lock" type sandwich bag with a glob of grease. Close the bag, then mush the grease all around & through the bearing by working the bag from the outside.

The advantage is in cleanliness - if you're working outside (as I usually have to), you won't be getting grit & dust into your nice clean bearing & grease, and your hands won't get as messy. Also, you can leave the bearing inside the bag until you're ready to reinstall it. (Fewer nasty words resulting from dropping your freshly packed bearing on the ground, too!)

Use the name brand bags - generic ones sometimes split at a seam when used in this fashion, and often don't have heavy enough plastic. Freezer bags, on the other hand, are sometimes too thick, making them unwieldy.