There are lots of tools and techniques that can help when designing a garden on plan:
1. 3D software is ideal - because you can take a 2D plan and see what it would be like to stand in your finished garden.
The drawback is that there is a fairly steep learning curve with most programs, and the programs themselves can be quite expensive.
2. Photographs and tracing paper is a great way of visualising how your garden will look. This is really helpful in seeing how big you need plants to grow to hide ugly views, or to see how much space plants are likely to take up in the garden.
Drawbacks are that you still need a fairly good eye, and a basic ability to draw perspective.
3. Overhead plans are often the easiest to get right. Also, if a design looks right on plan when viewed from above it will usually look correct in reality.