OK like all have said, definitely use the 4th finger for black notes, and the 5th finger for white notes.
My hands are a little bit bigger than normal, so I actually sometimes find myself using sort of both the 3rd and 4th finger at the same time on black notes. Also, on repeated octaves (as in the same note over and over at a fast speed) I put all three of them on the key: the middle, 4th, and 5th fingers... it seems to help.
What I HAVENT seen anybody say yet (very surprisingly...) is that you need to almost completely
eliminate all vertical motion. Your fingers should be touching the keys at all times while playing fast octave passages. When you practice slow, your 5th/4th/whatever finger you're using with the thumb should almost just sort of roll from one note over to the next one. This makes your octave technique so much more efficient, and you can speed up a lot that way.
Another thing I find very helpful, is this: put your hands on your knees. Now, keeping your arm, hand, and everything totally relaxed, raise up the base of your hand (sort of your wrists I guess), and then slap your legs with them (but your fingertips should always remain on your knees), in a tremolo type motion. Do this as fast as you can, while keeping everything completely relaxed. Now, try making octaves feel that way (rather than playing them "with your fingers" think of only making the motion with the base of your hand while your fingers happen to be in an octave position). In fact, practice whatever octave passage you're playing, and play each octave with the base of your hand like I just said, and smash all the notes in between the octave with your hand (like a cluster technique sort of). Always keep everything relaxed (I can't stress this enough
). Now, take away the notes from the middle, and just play the octave notes, making it feel that way.
The last thing I think about, is playing the octaves without your thumbs. I mean, not REALLY without your thumbs, but only think of playing with your pinky/4th finger/middle finger. In other words, put most of the weight on the outside of the octave. This not only helps you to stay relaxed, but it forces you to voice the octaves well too.
(this is all stuff I learned at a piano camp I went to this summer... One of the master classes was focused on octave technique and I feel like I really gleaned a lot from it... I can probably play octaves at least 50% faster now than I could before)
Hope this helps!