As Slaanesh's title is the "Prince of Excess," I believe that the fundamental qualities of its worship involve, well, excess.
Eating the most amount of food, of the most expensive qualities, of the most variety
Having the greatest amount of sex, with the greatest assortment of people, enjoying the greatest amount of pleasure
Write the greatest music, with the perfect combination of notes, the perfect shifts, the perfect length, and to train such musicians to give the perfect performance.
Creating the finest piece of art with the finest tools, the finest canvas, the finest paints, and with the finest detail imaginable
To obtain and achieve such limitless excess is simply achievable given the mortal bounds of mankind, and can only be obtained by casting aside the petty rules of morality in order to obtain this goal. But, even if the material can be obtained, there is still the act of the desire, which humankind, as mortals, will never be able to perfect.
In glory to Slaanesh, his/her cultists give praise through their aspirations for perfection in their desires, to shape and get their craft to the point of perfection, no matter how impossible it may be.