Water plants with a diluted seaweed solution to ensure good contact with soil and to encourage roots to grow.
Harden off plants by leaving them outside, sheltered from hot sun or wind, for increasing periods of time for about a week before planting. Plant early in the day or on a cloudy day, and give plants adequate space.
Do not till or dig excessively because you will destroy soil structure and kill microorganisms. Dig or till when first preparing beds, then just add compost on top and let the critters work it for you.
Marigolds, alliums, rosemary, mints, radishes, and many other plants are supposed to repel certain insects, but they attract others, too. There's so much serendipity in the garden, it's hard to prove which plant does what. But have fun experimenting!
If flea beetles, cabbage moths, or other insects are a problem in your area, protect transplants with floating row covers immediately after planting. (If you wait, the insects will lay eggs and you will be incubating them under the covers.) Remove covers when plants flower so pollinators can do their job.
Water regularly -- maybe with a drip irrigation system -- but don't over-water. Stressed plants are more likely to be attacked by insects and less able than sturdy ones to withstand damage.
Pick eggs of predators, including potato beetles and cabbage worms, from the undersides of leaves, but familiarize yourself with the shape and color of eggs. Lady beetle eggs -- ladybugs to most people -- look much like potato beetles, for example, but leave them alone to hatch and they'll eat the beetles eating your plants!
Work plenty of composted leaves and vegetable matter into the soil to improve its tilth and fertility. Good soil is alive with worms, beetles, springtails, fungi, bacteria, nematodes, and many other organisms that will help your plants grow.