Consider the difference between midi possible and reality possible

If your composition is based on a theme or motif, it is recommended that you state the original at least once in the beginning of the composition before transforming it 

Always plan out your composition first before composing the first bar. You can always adjust or change your plan while you are working on the piece

Do NOT throw in all the extended techniques you know just to achieve a "complex" sounding composition, as it would be meaningless. Extended techniques are useful for making subtle changes/variation to tone colour or to enhance a particular effect the composer is after

Research the instrument well before composing. Be careful of pitch range, tone colour of registers, breathing, fingerings, etc.