The best method I've found, for my budget and time, is hand-drawn plans. Grab a compass, ruler, markers, and pens and have at it!
It gives you much more freedom than any program I've found as long as you've got some amount of patience for drawing. I usually do a quick draft to figure out the structural plants and colors and then scale it up to a few feet wide sketch for the actual install.
In my experience, clients like to see some 3D renders as well so I use GardenPuzzle to showcase the important aspects of the landscape from a couple of different perspectives.
Good luck to ya!
EDIT: I just realized this thread was bumped from July, so you probably figured something out by now!