Personally I use my thumbs on black key arpeggios, but anyhoo, going slow is the way to go about it. Press down into the keybeds to "train" your fingers where they are suppost to go. You can lighten it up, once you add more speed of course. What really helped me is instead of just playing strait--all the way to the top of the 4 octaves and then back down--my teacher recommended that whenever I made a mistake I should go over and over that spot up and down and up and down say 5 notes in either direction of the slip. Then my fingers learned the notes rather then just being dependant on the familiar motion of once all the way up and once all the way down. It's tricky learning to run up and down within an arpeggio, but very helpful. Try whenever you pass the piano or when you first sit down, just doing some of these arpeggios even once. This way you'll learn to do them cold as well as getting in a bit of extra practice. Alternate rhythms is a good idea, but I think someone already mentioned this. So yah, practice lots and go slow. Remember, you're not hindering yourself by going slower, because your fingers are still being trained. Actually, the slower you go, the longer your hand and arm muscles are in each position, so the more they get 'trained.' They were a kill for me to, but I'm finally taming them.