Mario and Luigi Cheats and Glitches
Well, seeing as this game is less glitchy than Super Mario 64 DS and Mario Kart DS, I will only say the tactics I have known to work, and well.
Infinite coins (discovered by many, many players, and verified)
To get infinite coins, go to Thwomp Caverns and there are some areas where you have to throw the babies into a tornado, and make them spin into the coins. Keep throwing them down and collecting the coins, then walk left with the adults to re-group with the babies and repeat ad infinum.
Go through Toad in Princess Peach's Castle
To go through Toad, jump over him only with Mario and keep walking. Often, Luigi just goes through him like he don't even exist!
Tip for bean collecting
In the modern Princess Peach's Castle, the two rooms on the far right and left of one of the upper floors have bean spots. Now remember where they are, and go there in the Shroobified version of the castle in the past. There will be bean spots in the exact locations as in the future place. While not a cheat, this is also helpful to remember (and that all rooms in the modern castle have a past version parallel).
Not a cheat, but useful:
I would recommend the Ulti Free badge for one character, but not both (unless you can afford it). Equip the Hit free badge to the other character (faster is preferred, but I used the slower one anyway) and then you will have infinite Bros Items for both if you never get hit! Very useful for low level speed runs and the like.