The most general answer is to keep them cool and dry. There is a fair amount of variation from one chemical to another. Some things, like dry glycin) just seem to deteriorate over time. Others, like sodium carbonate, form hard to dissolve lumps. Some are photosensitive (notably silver and iron compounds, but also others) and need to be protected from long exposures to light. In general, keeping stuff in the dark (more or less) is a good idea. Protecting developing agents and other reducing agents from air also makes sense. Not buying more than you will need in the next year or so makes sense, too.