New Super Mario Bros.Wii is definitely a hard game and there's nothing more frustrating then seeing the "Game Over" and having to replay the last few levels. There's a solution to this and it's Infinite Lives! There are 1UP tricks for single players and 1UP tricks for multiplayer. The multiplayer way seems to be the quickest way to rack up the 1UPs. Here are the different tricks you can try to get infinite 1UPs:

World 3-2: Infinite 1UP trick

This is how you can get infinite lives on World 3-2 of New Super Mario Bros.Wii. First you'll need a Yoshi. He can be found at the beginning of the level all the way up in a tree. Head towards the mountains with the Bullet Bills and in between the two mountains is where the infinite lives happen. Jump on as many Bullets as you can, after a few all the following Bullets you jump on will be 1UPs. Check out the video of how it's done:

World 4 Final Castle - Infinite 1UP trick

So if you can't get the last infinite lives because your fingers won't mash correctly, then you can call a friend over to try this two player 1UP trick:
quote from the YouTube page:

This must be done on the final castle of world 4.
You need at least 2 players to do it and one of them must have ice power.
At the spot in the video where the fence has a long rectangular switch panel, kill all the koopas but one.
Have one player get on the switch panel and wait until the koopa is also on it.
The player on the panel hits the switch causing the panel to flip with the koopa on it.
While is it flipping, the other player freezes the koopa. You have to freeze him while he is on the switch panel and while it is moving. If you do it correctly you'll get the result seen in the video.

World 2-5: Infinite 1UP trick

This is the laziest and probably the easiest way to rack up 99 lives.

A little variation of the same trick:

World 2-2: Infinite 1UP trick

Here's another 1UP trick that you can do on World 2-2. It takes a little time, but it sounds like fun dad/son thing to do. :)