My observations and suggestions will be general, but I believe that they will prove useful in most cases.
1. When commissioning a work, to the extent possible, know what you want, and for whom.
a) Be clear about the medium (orchestra or chamber orchestra, wind ensemble or brass band, SATB a cappella choir or accompanied choral group, if a non-standard chamber group, list the instrumentation available, etc.)
b) Be as clear as possible concerning the performance level of the work you want. The technical demands that may be handled easily by a professional orchestra may prove too difficult for a community orchestra. Do not hesitate to discuss the strengths and weaknesses (if weaknesses there be) of the performing ensemble with the composer.
2. If length is an issue – for example, the piece must be no more than five minutes long – make this a condition of the commission. The same holds true with minimum lengths. Too often, lengths are mentioned as if they were suggestions rather than necessary conditions. There will be times when specific lengths may indeed be irrelevant, in which case the piece is the length – as a unique entity – it needs to be given the material, form, style, etc. Otherwise, state the length.
3. It is important that a clear and definite deadline for completion of the work be a condition of the commission. Negotiate a deadline that allows time for the commissioner(s) to examine the work and make requests regarding fixes or minor changes to the score (I generally give a three month window whenever possible.) Allow time for the extraction and editing of parts.
4. A great new piece should be a work of art, but commissioning is business. Do not hesitate to negotiate with a composer. The more that specific issues can be decided at the beginning of the writing process, the easier it is for the composer to do his work. Clearly stated external requirements of a piece make the project more defined, and thus relieve the composer of unnecessary choosing among options. Only the amateurish will complain of reasonable commission requirements.