Blight is something of a nightmare. It thrives in warm, damp conditions and can spread tens of miles on the wind, so it's largely unavoidable. This is the first year in a long time that I've not had it to some degree here (so far), which I assume is down to the very dry weather. My potatoes always suffer first because they're outdoors, but it does eventually get to the tomatoes in the polytunnel too. Fortunately that's usually not until very near the end of the season when we're just trying to get the last few fruit to ripen.
The blight fungus actually requires a live host and from memory can't survive in any form (spores or "live" fungus) without one for more than a few days, so composting plants that have it really shouldn't be a problem. I've read suggestions that where potatoes are concerned, the larger problem is "volunteer" potatoes from previous years' crops that provide a host for the fungus over Winter and then spread it again when they sprout in the Spring. For that reason I've been removing all signs of volunteer plants this year.
Where potatoes are concerned, I think growing resistant varieties is probably the only real choice unless you just learn to live with it. Farmers have other options using chemical controls, but not ones that are available (nor even viable, really) to the rest of us. In the case of tomatoes growing under cover seems to be the best choice though there are blight-tolerant and blight-resistant varieties if that's not an option.
Watering when the air is still might help, as the fungus needs a period of time when the plant is wet to infect it, but all it takes is a rainy and windy summer day and whatever precautions you've taken may well be rendered useless
James