OK, you have a few options here:
1) If you and a boosting partner both have a turbo pad then set up a player match and set the 'A' button to turbo. This way, the time will run out each game, and rematch will be selected straight away - So no loading times between games etc etc. Easily the best method for boosting.
2) If you and a boosting partner haven't got a turbo pad, then create a 2 player lobby and just leave the game running, the loading times will hinder the speed of which your time increases, but it's the second best way of boosting.
3)If you haven't got a boosting partner, then go into a lobby (the more people the better) and press 'X' to spectate the games. Though you may want to check on it every so often incase everyone in the lobby has left.
I have noticed that sometimes after being in a lobby, my time online would sometimes go down, so If i boosted in a lobby, I would go into a quick ranked/player match afterwards, to make sure that didn't happen.
Obviously it is much more fun to get this without boosting - But just giving people another way.