I usually like to use the most I can out of an item. Funny thing is, I never got this item you speak of. But if I did, I would use it like this:

Use the second one 1 second after the first one. The third one 1/2 second after the second one. The fourth one 1/4 second after the third one. The fifth one 1/8 second after the fourth one and so on. The next one waiting just half the time of the previous one. :D

Then I would be happy because I know I used all boosts I could use. :P

I kinda thought the same way when i used them on mario kart 64 since to me i thought mashing it would have made me faster but IMO it didnt. Until i used it for the first time on wii it made me faster cause i can see it (zooming right through the other racers. Once you use them you gotta use them as much as you can, that it unless your making thoes sharp turns then dont use them too much or you'll hit a wall or fall off the track (believe me i know :( ).

I believe the issue is whether there is a slight stutter each time you use it, so that you don't immediately hit the top zooming speed. It seems to me that, for certain games in the series, Kart 64 most notably, there is in fact a very palpable stutter on each subsequent press of the button, such that jamming the button may actually slow you down, and the best timing is to be found in the tradeoff between that initial stutter and the loss of speed that occurs if you wait too long between uses. I'm not absolutely certain, however, how this may have changed in later games in the series; I still feel that jamming the button as fast as you would, say, in a Mario Party mini-game is never ideal with a golden mushroom, but I've not spent enough time gauging it in the Wii version in particular to be certain.

 Thats what i was thinking but i couldnt think of a proper word for it to put in my post.

Spread it out... I'm fairly certain that when the boost starts it slows you down a bit, so mashing it actually slows you down more then speeds you up.

At least that was the case in Kart 64.

I wish Mario Kart Wii had a speedometer like Mario Kart 64 did.