First of all, study the instrumentation, including the basic range, techniques and articulations. Study them along with musical examples, and study some existing repertoire, to gain some fundamental knowledge of those instruments. If you are writing for a non-symphonic instrument, suggest to also study some historical context together with both traditional (historical) and contemporary repertoire.
Second, if you want to write something compelling, ask yourself what may be compelling to you and find out the details. Imagine and explore your ideas with the instrument's syntax. Never hesitate to consult and work with those players if you are not very experienced or familiar with the instruments.