If there was only one piece of advice I could give you that would yield the greatest benefit it would be to plan you rock garden project from start to finish. The results will be far superior.

As with any task where a place is to be changed you need to remove the old before the new can be installed. Make sure all the debris is removed and any remaining plant life is taken out. Just like a painter of great art you need to start with a clean canvas.
One of the first decisions you need to make once the area of your property is staked out is what kind of rock will work best for the planned project. You might want to use those that are similar and otherwise you may want to use those that you like best and create a collage effect.

Ok, now we have some ideas that we want to use for our rock garden and the next thing you will want to do is place the largest of them where they go then go back and work on the base layers of small pebbles before you complete the task by filling in all the blanks. By choosing the right combination of rocks that are of the larger kind you will make a garden that is appealing and interesting at the same time. Believe me it all works together to look great with enough forethought.

The next step to a superior rock garden is the rock bed of filler that will consist of interesting stones or rocks that will serve as a canvas to create your masterpiece. With the background in place you can start to fill in with interesting plants laid out in patterns to draw attention to the places where you want. Beyond that, shade of many sorts will finish off the area offering the contrast to hold ones interest. There is another option that is not natural but some people will find interesting, the use of fountains and other manmade features but again it is personal preference.

The use of plants in a rock garden may seem to be a contradiction but even most dry places there are some plants and no one said we were going to duplicate nature in this garden. Find the ones you like and think will fit your garden and lay them out to see how it will look before you get started planting.

As the last step you will want to add elements that are not necessarily rock related. Things that you can set outside permanently. I find it best and most attractive to use rustic things that I have picked up from estate sales that no one else really wants. They look great but you have to put in the time to find them. Good luck and happy gardening.