The other thing to consider when planning your plant placement is that it is often best to use groupings of at least 3 of the same plant together. One plant alone often does not have enough impact, where a grouping of 3, 5, 7 or more will have good impact. Odd numbers tend to look better than even numbers. This is especially true of smaller plants where groups are necessary to have impact. Short plants can be used in long narrow plantings to create borders on the edge of a bed.
There is an exception to the plant 3 or more plants rule. In general, if a plant is large enough, think shrubs or large perennials, it can hold it's own without being grouped with other plants. Usually, only back of the border plants can stand alone. Scroll back up to see photos of borders.
Island beds work on the same principles as borders, but rather than having the taller plants in the back. The taller plants are in the middle of the bed or centered on the anchor plant. In the design below, the bright blue dot is the anchor, the brown are the tall plants, the pink are the medium plants, and the dark blue are the short plants.
You will note that the plants are grouped in drifts with the taller plants in the middle of the bed and then getting progressively shorter as you get toward the edge. Your design doesn't need to be rigid, you can see above that some medium sized plants come to the edge of the bed and some short plants are right next to tall plants. The tall to short progression is simply a rule of thumb, not a hard and fast rule. Scroll back up to see photos of island beds.
Let's talk now about the two styles of garden. Gardens generally are either formal or informal. Formal gardens tend to use distinct geometric shapes for their layout ; circles, rectangles, triangles or long straight lines. Plant spacing, color, and layout are all very precise. Here are some examples of formal gardens:
On the left is a circle knot garden, in the center is a formal garden with lots of color, but very geometric shapes, and on the right is a formal garden with a clipped boxwood hedge to create the form of the beds. All of these photos were taken at the Missouri Botanic Garden.