Good subnet planning prevents painful renumbering later. Plan for growth and keep allocations organized.
Reserve space. Don't allocate your entire address block. Keep % free for growth.
Use consistent sizing. If server subnets are /, keep them all /. Consistency simplifies firewall rules.
Document purpose. Every subnet needs a description: "Building A workstations" not "VLAN 50."
Group by function. Put servers in one range, users in another. This makes ACLs cleaner.
Plan for summarization. Allocate contiguous blocks so you can summarize routes.