This one is fairly easy, I like to find in books and magazines pictures of gardens I like and jot down the plants that I can name and attempt to figure out a way to make my yard look like theirs. The plants I do not know or ones that do not work in my climate I replace with ones I already have or know.
I also like to use companies the sell plants for their garden plans they put out for you. If you have a plant already that you want to create a new bed with you can look it up on a sellers site as if you were going to buy it. Then look around that site, almost always there will be a tab on that plant used in a design. At the very least it is giving you a list of other plants that work well with your selection and also require the same amount of light, drainage and soil content!
One great tip that I once read is the way to making a garden look spectacular is to place plants close together but also to put different shaped leaves together to hi light each plant. Here again though these companies do this for you in order to sell package deals with plans, many times I find great ones in the back of magazines or even coupons trying to sell you the set for hundreds. You can do it for almost free.
The best way to copycat in the garden for almost free though is to propagate. If you know nothing about it you can learn a lot from places like Gardenweb where in the forums people discus it at length. Like cooking though you have to consider how things got their start. Large home centers are not what make gardens beautiful it is people and all the fun in gardening is from the little and successful experiments that you try. I always remember my father like a bee working his plants sometimes even pollinating to try for new flower colors. Sometimes collecting seeds and packaging them for the following year. Which brings me to the easiest way to propagate… seed collecting. The rule of thumb is seeds are typically behind the flower. That means that if your plant is flowering it is going to create a fruit which is in fact the seed. sometimes we eat the surrounding part of the seed but for flowers (except say sunflowers) we tend not to eat them. Either way they are there, when the flowers are drying and dropping off its time to look for dark dried up pods of seeds about where the flower was. Say four O'clocks they are a large seed just sitting inside the flowers old spot. In zinnias, black eyed susan and marigolds, its the head of the flower itself. This tidbit means that if your diligent you only need to buy a plant once and you can duplicate it as many times as seeds you can collect ahheem and time of course. Another propagation method is by cutting this one could be anywhere from really easy to impossible somethings, many things will send out roots in just water. Think about the fact that plants only goal is the same as animals to survive and reproduce, therefore if you take a branch off some plant and stick it in water (new water each day, not stagnate water) it will send out tap roots. You want to try it? its fun, the easiest is Coleus it will happen in about 3-4 days and as soon as you see those little roots put it right into the dirt. There you have it whole new plant. Sometimes propagating this way is as easy as just sticking the plant into the dirt, Sedum is best for this break off a piece and shove it in the dirt, the following year -- huge plant!! Forsythia all you have to do is pull a branch to the ground and place a rock on top of the branch you can extend one plant across the yard this way. Imagine what you can create with this small amount of knowhow.