Never spray on a windy day, to prevent spray from drifting. Keep in mind that even on slightly breezy days, the higher you spray the greater the chance the material may unintentionally land on you or some desirable plant.
Spray during the cooler hours of the day to reduce the risk of scorching plant foliage due to rapid evaporation of material. Don't spray if rain is in the forecast.
Spray just to the point of run-off, no more.
Test your sprayer using plain water before beginning any job.
Use the correct dilution for your particular job. Mix only as much as you'll need and follow manufacturer's directions for mixing and proper disposal of leftover product. Never combine one product or chemical with another.
Scrupulously clean your equipment after each use. If possible, have several sprayers and dedicate each to the exclusive use of a particular pesticide or fertilizer. Mark each sprayer with its name. (Weed killers are particularly hard to clean out, and even a trace amount could harm desirable plants).