Hierarchy: Have few focal pieces and build your garden around them. Many times, depending on the size of the place, one focal piece per area is plenty. More than one will become busy and start to look like your neighbor's place who has gnomes and stones with sweet sayings on them lining the walk. It is better to do one 'big move' than many little choppy bits.
Height Layers: Know the mature height or the height you intend to maintain your plants and layer them into the landscape accordingly, giving rhythm and balance.
Contrast: Color among other attributes can be very powerful when used sparingly. Take note of the colors of green that work together as well as bloom colors. Also, make sure the colors work with your structural backdrop. Red, orange and pink do not work together! Sometimes it's best to use one or two colors and mix variations of the same color throughout.
Texture: This is something you probably won't be able to do from images. Go to the nursery, and pull leaves from the different plants you are interested in and put the leaves together. Do the textures work well together or are they fighting each other? Leave size and pattern also play a big role. Spiky then leaves aren't typically going to go well with soft, lobed leaves.
We will get into many other design aspects in later posts. I just thought it was good to get some basics down before jumping right into plant species.
I hope you enjoyed!