I finally did it today. After many hours of playing this game, I've completed it 100%. Since there are a few ways to make that goal a bit easier to reach, I thought I'd share my findings. Feel free to add anything you know. 
Beat levels using all characters quickly
Requirements: extra controllers (Wiimotes are fine)
1. Play a level alone with any character you like
2. As soon as you get near the end of the level, add other characters to the game on the other controllers
3. Line up all characters around the Goal Pole, then quickly jump to it with each character on each controller
4. Now you'll need to play a level only twice vs. five times
If you practice this properly, you'll need about 20 seconds on the timer and you can make 3-4 characters finish a level on a single try. Note: as soon as one character touches the Goal Pole, you only have a few seconds before the level finishes so act quickly. To make this easier, ask someone else to control a character for you. In levels without a Goal Pole (e.g. Mystery House levels), add other characters to the game just prior to leaving the final room (at which time the timer doesn't count down anymore).
Max out your lives (up to 1110 or 3 crowns) quickly
Well-known method in World 1-2:
Even quicker method in World 5-1:
Use the cat suit for a guaranteed Goal Post hit at the end of a level
I always went back to World 1-1 (Super Bell Hill) and collected two bells to transform my character into the cat suit and have a second cat suit as spare. Since almost every level in the game can be played with the cat suit on, just make your way to the end of a level and climb to the top of the Goal Pole in the cat suit to hit the Goal Post. This is great when you play as a character with low jumping power (e.g. Toad) or slow speed (e.g. Luigi).
Get a sparkling 5-Stars save file
The conditions to unlock stars for the save file are as follows:
1-Star: Complete the game until the 8th World.
2-Stars: Acquire all Green Stars until 8th World.
3-Stars: Complete Star World, Mushroom World, and Flower World.
4-Stars: Acquire all Green Stars in the first 11 Worlds.
5-Stars: Acquire all Green Stars, Stamps, beat all the Worlds with every character, and hit all the Goal Posts.
If you don't have sparkling stars, it means you've completed a level using the Invincibility Leaf. To check for levels on which you've used the Invincibility Leaf, go to the Map, check your records and look for levels with a score of "00000". If you have levels where the game hasn't recorded a completion time, that means you've failed at least once when playing that level. Play again without using an Invincibility Leaf or dying and you'll get complete records.
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So that's as much as I could figure out by playing the game. It has tons of replay value, great controls and variety, and is one of the best 3D Mario games to date. Anyone else completed the game and liked it all the way through to the end? If I had to nag at something, then it's probably the bland story. Super Mario Sunshine in that regard is still the better game IMO.
Beat levels using all characters quickly
Requirements: extra controllers (Wiimotes are fine)
1. Play a level alone with any character you like
2. As soon as you get near the end of the level, add other characters to the game on the other controllers
3. Line up all characters around the Goal Pole, then quickly jump to it with each character on each controller
4. Now you'll need to play a level only twice vs. five times
If you practice this properly, you'll need about 20 seconds on the timer and you can make 3-4 characters finish a level on a single try. Note: as soon as one character touches the Goal Pole, you only have a few seconds before the level finishes so act quickly. To make this easier, ask someone else to control a character for you. In levels without a Goal Pole (e.g. Mystery House levels), add other characters to the game just prior to leaving the final room (at which time the timer doesn't count down anymore).
Max out your lives (up to 1110 or 3 crowns) quickly
Well-known method in World 1-2:
Even quicker method in World 5-1:
Use the cat suit for a guaranteed Goal Post hit at the end of a level
I always went back to World 1-1 (Super Bell Hill) and collected two bells to transform my character into the cat suit and have a second cat suit as spare. Since almost every level in the game can be played with the cat suit on, just make your way to the end of a level and climb to the top of the Goal Pole in the cat suit to hit the Goal Post. This is great when you play as a character with low jumping power (e.g. Toad) or slow speed (e.g. Luigi).
Get a sparkling 5-Stars save file
The conditions to unlock stars for the save file are as follows:
1-Star: Complete the game until the 8th World.
2-Stars: Acquire all Green Stars until 8th World.
3-Stars: Complete Star World, Mushroom World, and Flower World.
4-Stars: Acquire all Green Stars in the first 11 Worlds.
5-Stars: Acquire all Green Stars, Stamps, beat all the Worlds with every character, and hit all the Goal Posts.
If you don't have sparkling stars, it means you've completed a level using the Invincibility Leaf. To check for levels on which you've used the Invincibility Leaf, go to the Map, check your records and look for levels with a score of "00000". If you have levels where the game hasn't recorded a completion time, that means you've failed at least once when playing that level. Play again without using an Invincibility Leaf or dying and you'll get complete records.
----
So that's as much as I could figure out by playing the game. It has tons of replay value, great controls and variety, and is one of the best 3D Mario games to date. Anyone else completed the game and liked it all the way through to the end? If I had to nag at something, then it's probably the bland story. Super Mario Sunshine in that regard is still the better game IMO.
