Choose musical collaborators for professional compatibility and complementary skills, not friendship
The right musical partnership produces better and/or more music than working alone; the wrong one is more painful than solo work. The primary selection criterion is professional compatibility (work ethic, musical and technical skill), not social compatibility — extending a friendship into a professional relationship risks destroying both if the professional side fails. Seek partners who counterbalance your weaknesses: the classic producer-plus-vocalist pairing works because each supplies what the other lacks; more generally, find someone strong at a production stage you are weak at or dislike (e.g. arrangement). Remote and even anonymous collaboration is especially easy for electronic musicians, who need no acoustic treatment or mic setups. Whatever the arrangement, agree explicitly on workload, money, ownership, and creative rights as early as possible to avoid later misery.
Examples
You struggle with arrangements but excel at sound design. Rather than partnering with your best friend (a mediocre producer), find a collaborator who is a strong arranger. Before starting, write down how royalties, credit, and final-cut decisions will be split.
Assessment
Identify one production stage you are weakest at. Describe the ideal collaborator who would complement that weakness. List the four things (workload, money, ownership, creative rights) you would agree on before starting, and why each matters.