I appreciate how DarkTyphlosion's answer is a great competitive guide, but I think that the question was intended for in-game use. Also, seeing as you only have four badges you won't have access to the day care for a while. While my method isn't so great as a grinding exercise, it works. Get the soothe bell for your Pokemon to hold from an old lady in a building on the left hand side of Nimbasa City and get a massage from the lady on the first floor of the building opposite the Castelia City gym every day for your Pokemon that you're Happy Training. The main training occurs on route 19 (after Aspertia City) Ride through the second/middle patch of grass on your bicycle over and over again, defeating all the Pokemon you encounter. Use all of your pp then call Bianca on the Xtranceiver and look at friendship. If she says that your friendship makes her jealous, you're ready to evolve!
Although vitamins help, they tie in with effort values and aren't necessary for in-game play