There is a glitch in EarthBound (removed in Mother 1+2) where the Rock candy can be used multiple times. To do so, one of Ness' party has to have at least one condiment, and a Rock candy in the last spot in their inventory. Entering a battle and using the Rock candy on anyone results in the Rock candy remaining and only the condiment being used. Both Sugar and a Jar of Delisauce double the regular effect and are therefore the best to use in the glitch. However, this glitch can fail; adding anything to a statistic of 255 results in it being reset down to zero, and odd effects have been noted with having an HP or PP stat increase past 999.
The best way to get incredible stats is to have one use the Rock candy multiple times in battle. The more Sugar Packets, the better the stats. This trick works especially well before a level-up. It is also possible to set up more than one Rock candy to gain more stats in a single level-up.