Sketchup is the way forward!
I recently attended a Sketchup training course organised by the Society of Garden Designers (SGD) as part of their new Designer Develoment Days (DDD). The course was held at The Greyhound Coaching Inn in Lutterworth by Paul Hensey.
The course was a great way to refresh your memory of Sketchup or as a starting point to use CAD more in your portfolio of techniques to design gardens.
I really enjoyed the course and it has reignited my interest in using my computer more to create quick visuals rather than being restricted to my drawing board most of the time, through habit and ease more than anything.
I think I will always sketch drawings out by hand first even when using Sketchup though. Nothing beats pen and paper to get ideas flowing but computers are great for accuracy and making quick amendments.