To beat the staff at Nintendo that set time challenges on Mario Kart Wii the tip is to:
1. Always ensure that you have the best type of vehicle for the course you are competing in eg. if you are doing Luigi Circuit you will want a fast car, where as if you are doing a track with lots of twists and turns such as Deffino Square you will want a light weight bike that can accelerate quickly and steer with ease through the corners.
2. Get to know the course before you try the time trials. Usually there are shortcuts that the Nintendo people don't use that give you great time advantages. Also if there is more than 1 way of completing the track such as Delfino Square try going both ways to find the quickest way.
3. Use a bike for short twisty circuits. The main reason being if you do a wheelie by yanking the wii remote suddenly towards you, you will go faster.
4. Look around for jumps because if you take it whilst you are up in the air, shove the wii remote towards you and you will get a boost. (there is one on the grass towards the end of the circuit)
5. Take every advantage you can. Use the mushrooms to boost you wisely. You can do this by pressing "b" when you need a boost. (It works better on tarmac, do not use it to get off the grass)
6. Follow the record holder sometimes they use shortcuts that maybe you can use!
7. And finnaly keep practicing!!!!