Yes. From the clues we got, it seems as though they were BFF up through fifth year, and probably stayed partners in Potions even after that; Lily was one of Slughorn's favorites, and Snape was always a genius at it.

It won't come easily for Harry, but now that he knows what Snape is *really* looking for (someone who can understand the whys and wherefores, not just follow a recipe) he'll try to make an effort. =)