The general rule of thumb with regard to heights is perfectly sensible, that is plant the shorter plant at the front and the taller ones at the back.
This can, however, produce a very regimented picture, rather like a school choir, so try to vary this slightly by bringing some taller plants forwards.
This will break the formality and also obscure some plants, so that they are revealed only as you walk along the length of the border, making it more interesting.
There are some plants, such as Verbena Bonariensis, that, while tall, are also very sparse and wiry.
Such plants can be planted towards the front of the border, where they will create a misty effect as you look through them to the plants behind.
But how do you work with plants with different shapes? Finding good ways to work with plant shapes is more complex than working with height.
Learn by looking at other gardens and deciding what you think works well.
Contrasting shapes certainly make a border more interesting.
So, the sword like leaves of an iris contrast well when planted in front of the giant leaves of a Rheum or ornamental rhubarb or the filigree foliage of Myrrhis or sweet cicely.
Likewise, frothy grasses or the sword like spikes of an iris work well with the more substantially leaved hostas.
In some garden, sword like, spiky leaves draw the eye up and away, and in doing so create a sudden burst of energy in the middle of the border.
The tall stems and rounded flower heads of the angelica create a distinctive, cloud like image in the border that sets it apart.
The ever popular pampas grass, Cortaderia, is a particularly stately plant and is extremely eye catching, whether grown by itself or as part of a border.
Salvia sclarea's upright nature makes it, quite literally, stand out in the crowd, creating a focal point in the border.