Botrytis is rife with insufficient ventilation and starts on dead or dying material, so maintaining a good air flow and making sure you take out any dodgy looking leaves, stems, flower buds or fruit immediately will help a lot
Also, not overcrowding plants helps with air flow, and I always feel it is better to have fewer but healthier and more productive plants in an under cover space than lots stuffed in
I have learned this the hard way, and so now, when given un unexpected gift of a tomato plant or two (or more!) but have already got the number I grew to fit the space, plant these extras somewhere outside to take their chances
Also, if I have had to chop up any mouldy looking stems etc, follow the RHS advice about dipping the secateur blades in very dilute bleach, which I keep in a cleaned out old screw top container than had powdered fabric whitener in it. It last all season.