Have 28 P-Wings
Complete the game, then press Start at the ending screen. The next game will begin with 28 P-Wings each for Mario & Luigi.

Warp Whistle to World 8
If you use a Warp Whistle and then use another one while in the area where you select where to warp to, you will be able to jump directly to World 8. There are a total of 3 Warp Whistles in the game, with two being in World 1 and one being in World 2, you can get to the end of the game very quickly.