My cube consists of 2n cards of each color, 2n artifacts, 2n multicolored and hybrid cards and n lands. Therefore, 2n * 5 + 2n + 2n + n = 15n cards. n boosters can be made out of that pool, on average containing 2 cards of each color, 2 artifacts, 2 multicoloreds and 1 non-basic land. Having perfectly balanced boosters however, would make it too easy to figure the colors of your neighbors when drafting, so I add a bit of randomness.
I start by shuffling each of the 8 piles (5 colors, artifact, multi, lands) and placing them face down. Then, from each of the 7 big piles (5 colors, artifact, multi) I take 6 cards for each player and from the small pile (lands) I take 3 cards for each player. Then I put together one pile (of 6 or 3 cards) of each category, making a 45-card pile, hand it to a player and have him/her shuffle it. The player will finally split the 45-card pile into 3 15-card boosters.
No huge pile shuffling is needed for this method (in contrast with the "shuffle everything up" method), but it is slightly biased, since the contents of the first booster a player opens reveal them the distribution they may expect in the next two they open.
How do you make your boosters?