Negotiating leverage decays as the renewal date approaches. Eighteen months out, you can switch, delay, re-scope, or run a competitive process. At the deadline, you can sign.
Vendors understand this precisely, which is why renewal clocks and 'best and final' offers cluster near expiry. The calendar is a tactic.
Starting early lets you build the three things that actually move price: a benchmark, a credible alternative, and a clear walk-away. None can be manufactured the week before signature.
The discipline is organisational, not legal. Treat major renewals as programmes that begin a year and a half out, and the leverage takes care of itself.