I’ve read mostly in extract brewing that you should buy better yeast and use the packet that comes with it in your boil as a yeast nutrient. Could you also collect yeast from your yeast cake and use that as a nutrient too?
I’m actually working on making a yeast extract out of a yeast cake to be used as nutrient, but that is going to take some time… so I don’t know yet 
I like using Wyeast beer nutrient - stuff works great and isn’t expensive. It uses a variety of nutrient minerals and yeast, to make a fermentation cocktail the little buggers find irresistable.
edit: Actually, a blend of diamonium phosphate, vitamins, yeast hulls, and yeast extract.
