The key is not to overcook it! After 20-25 minutes at 180°C (160°C fan), the salmon should still be slightly pink in the thickest part - it'll continue cooking once you take it out of the oven. Use a fork to gently check the centre - it should flake easily but still look a touch translucent in the very middle. Overcooked salmon becomes dry, so err on the side of caution. If you've got a meat thermometer, you're aiming for about 50-52°C in the centre for that perfect medium doneness.