Every year my tomato plants get powdery mildew. What can I do now?

are you planting them in the same spot each year? If so, you may have a fungus in the soil...if you need to keep them in the same location, plant them in 5 gallon pails dug into the soil (put new soil in the buckets). If there is another spot that they could be planted, use that. Crops should be rotated every year to avoid over Using the nutrients out of the soil.

Keep water off the plant as much as possible, deep water either using a hose or soaker. It’s easy to do if you don’t get summer rain and has made a big difference with my tomatoes

Rotate your crops and you can avoid a lot of problems. Try to rotate them so they aren't in the same spot for at least a couple of seasons. Make sure you have plenty of circulation between the plants. I change mine each year. I always have them grouped in two rows of three with the largest cages I have. They end up growing together and supporting each other at the top, but for at least three quarters of the way up until they make their own canopy, somehow they get enough circulation to not have problems. I use soaker hoses to water my veggie garden so that the foliage stays dry. Occasionally I will rinse the whole garden down to get rid of dust, etc. when it hasn't rained for a while and try to do it in the morning so the leaves don't get affected by the sun on the wet leaves. I put mulched leaves and cut grass on the soil to keep in the moisture so that I don't have the surface wet all the time from having to water so much.

You have received some good advice, Joe, but I think you really might like to read how this woman grows her tomatoes! Go on the excellent blog: “The Art of Doing Stuff”. This (single) woman tackles anything and everything and is a mine of information, builds her own patio furniture, maintains a large community garden in the good weather, knows everything there is about raising chickens, etc, etc.