Properly preparing your soil is the single most important step to having healthy, successful gardens. Dry, sandy soils can be improved by adding plenty of organic matter, such as compost, moistened peat moss, or composted manure. Dig the area to a depth of at least 8 inches, preferably with a fork or spade. Heavy clay soils need to be opened by adding plenty of organic matter, along with perlite or coarse sand. Gypsum will also loosen the soil, but will take 1-2 years to make a noticeable difference.
Few perennials do well in wet, poorly drained soils.
Consider building raised beds or installing drainage tubing if you have a soggy garden area. The planting area must be free of perennial weeds, especially spreading types like Canada Thistle, and Couch or Quack Grass. Annual weeds are easily controlled by hand weeding. Mulching around your plants will help to control weeds, and will keep roots cool and moist. Choose a mulching material that is organic and weed free, such as bark, cocoa beans, or shredded leaves.