In areas where you need to keep spinning a cog to keep the fan going spin it just once and go to a place onscreen where you cannot see the meter. As long as you stay in an area where the meter is not visible it will never run out.

There are two tall rooms inside the Thwomp Volcano where you use the Baby Spin move to fly down while gathering coins. When you have got as many coins as you can switch to adult Mario and Luigi to go to the previous room and then use the warp pipe to bring back the babies. Use the Baby Spin move again in the next room and all the coins will still be there allowing you to do this numerous times to gain an infinite amount coins.