For vegetables you can use physical means like row covers and netting to prevent flying pests. I wouldn't use anything else until a problem appears, apart from normal garden care e.g. weeding, watching and mulching. The Vegetable Forum can give you further advice.

For fruit trees I think you'll get a better number of suggestions over on the Fruit and Orchards Forum. There are various sprays you can use to prevent specific problems. But they'll need to know precisely what fruit trees you have and where you are located.

I've been using Surround WP Kaolin clay crop protectant for several years with great success. The list of pests and diseases that an application or two protects against is impressive and I've found it to be very effective.

I also use it after the suntry, sunny summer settles in to protect my tomatoes, peppers, and squash against sun scald and heat stress.

Surround WP is listed for organic gardening and is OMRI approved.

Let's see.....here are some of the things I use it for: prevents egg laying of caterpillars, stinkbugs, leaffooted bugs, borers, reduces leaf miners, aphids, leaf hoppers, grasshoppers, Japanese beetles (which is why I started using it in the first place), bean beetles, powdery mildew, black spot, and more.