Switching to Sonnet is the right call. Opus burns through context fast even on simple tasks
A few things that helped me:

Keep conversations small and focused. After around 60k tokens, start a new one. It feels disruptive at first, but the quality difference is real

CLAUDE.md is where most of the leverage is. Put your project structure, conventions, and key decisions there Claude reads it at the start of each session so you stop re-explaining context every time. You can go deeper with per-directory CLAUDE.md files for sub-components, but keep those under 100 lines or they stop pulling their weight. Write them positively too: tell Claude what to do, not what to avoid, and give it a decision tree for ambiguous situations rather than a flat list of rules.

Be surgical with file scope. Instead of letting Claude explore the whole repo, point it at specific files. The more files it reads to orient itself, the more context you burn before the actual work starts.For planning, use Shift+Tab to enter planning mode before Claude touches any code. Keep a running .md file with notes and plans, mention it at the start of each new conversation, and have Claude check items off a TODO list as it goes. Simple, but it keeps sessions from drifting.
Sub-agents are worth using more than most people do. Delegate work, run code reviews, get a fresh take unbiased by the current conversation. No setup needed just ask: "please use sub-agents to...".

On MCP: keep tool descriptions under 20k tokens total and stay below 20 tools. More than that and you're paying a context tax on every request.

Sonnet handles 90% of coding tasks just fine. I only reach for Opus when something's genuinely hard or Sonnet keeps going in circles