Bordeaux Mixture. Buy it from a good garden centre and use it as it says on the tin.
To help with tomatoes Put a rain proof cover over them at the end of July so as to keep the excess rain off but to allow a good fresh air breeze blowing through them.
This is not really practical with potatoes so fall back on the Bordeaux Mixture. Study the leaves on the spuds about end of July and the moment you see them going brown, dig them up and use them. If you leave them you will lose them. Southportsmith.

Make sure that every tiny potato tuber is removed from your plot. That is the only part of the plant capable of spreading the fungus.
Choose a blight resistant potato to grow, preferably earlies.
Spray with fungicides at the appropriate times.

read this

http://www.blightwatch.co.uk/content/bw-Home.asp

and pray.

Hurry up if you want to use bordeaux mixture,it is a banned from sale substance from next year!

Don't try to treat it as fungicides merely inhibit blight and there are more and more resistant kinds developing. Blight does NOT survive in the soil but only on living tissue, so as said above, rigorously dig up all your old potatoes and as soon as you see a single 'volunteer' (potato shoot from old potato pieces in the ground) in the spring, dig it out and destroy it.
Blight is largely spread by the weather and in the wind, so if the weather is wet and humid, there is little chance you can avoid it, other than by growing your tomatoes under cover. The wet weather over the past three years has not helped.
The other thing you could try is a blight resistant cultivar like Ferline, but even these may succumb. In general, the large fruited varieties are far more susceptible to blight and you might find small cherry tomatoes at least give you some crop, even in a bad year.
The main thing to do is pray for hot, dry weather and blight will go away!