Protein fibrils are relevant not only in medicine and amyloid-related neurodegenerative diseases, but also as functional structures in material science or biology. The assembly of protein into fibrils can be promoted or inhibited based on the chosen environmental conditions and interaction with suitable components. We review here the key strategies for promotion and inhibition of protein fibrillation in both physiological and non-physiological conditions in order to create functional designs. The major variables discussed are solvent conditions, metals/ions, biopolymers, aromatic compounds, and surface active components. Due to bias in research directions, deeper investigation has traditionally been carried out for inhibition of fibrillation, but focus has recently shifted. Thus, while various strategies are presented on the breakdown of mature protein fibrils, emphasis is given to the approaches leading to increased rigidity and length of resultant fibrils. We highlight important areas in this field that require further development and promising lines of future experiments.