One of your biggest challenges of designing your own garden or landscaping will most likely come with choosing the right garden plants. Other than shape of design, this is where most people have trouble. While there is room for experimenting with plants, trials and mistakes can be costly.
Several times throughout this site you'll find me talking about the importance of having a Plant Encyclopedia in your library. Even though you can find a world of plant information in the garden plant databases, there is no substitute for a good plant encyclopedia. Being able to look plants up by characteristic on the spot or taking your encyclopedia with you to the nursery is a real plus.
A step further would be to have a Regional Gardening Reference for your specific planting zone.
Knowing and choosing the right plants is such an important step in creating a professional looking and cost effective garden or landscape design. Knowing a little bit about the plants you want to or can use in your area can save you a lot of frustration and money.
For some good ideas on planting schemes and design ideas see the free landscape design ideas gallery. And for hundreds of pictures from professional designers see the free landscaping styles pictures. Between the two galleries there are hundreds of ideas you can look at for hours.