Plants that aren't well nourished will be less likely to survive attacks by pests or disease.  Ensure your soil is nutrient-rich and able to retain moisture by supplementing it with organic materials.  In a garden bed, layer on compost or peat moss about an inch thick; it isn't necessary to work the material in, as it will naturally decompose.  Skip topsoil, which can be of questionable quality (it's often just the uppermost layer of dirt from some excavation).  If you're buying potting mixes for a container, look for those with compost, peat moss, vermiculite, and/or perlite.  Add Organic fertilizers to add  food for your plants.  My favorite all around would be  All Purpose Mix. 

When buying plants,  select specimens that are short and leafy with no or few blossoms.  You want plants to put energy into establishing roots, not fruit at first!  Choose 3" pots instead of six-packs; you'll get healthier plants that have had more room for there roots!  After planting, overspread the bed or container with at least a two-inch layer of natural material to help with retaining moisture and to prevent weeds.  This is especially important if you live in a hot arid zone like I do.