Inside the Thwomp Volcano, there are two tall rooms where you must use the Baby Spin move to fly down while gathering coins. Grab all the coins you can get, then switch to adult Mario and Luigi to go into the previous room and use the warp pipe to bring back the babies. Use the Baby Spin move again in the next room - all the coins will still be there, allowing you to do this numerous times to gain as much coins as you want.
In any area where you have to keep spinning a cog to keep a fan going, just spin it once and go to a place where you can't see the meter. As long as the meter is not visible onscreen, it will never run out.