Plant in groups
Apart from large plants, known as specimens, plants are best planted in groups of three. Bold splashes tend to look better than single plants. You can be forgiven for using single plants because space is limited as this is a common mistake. The impact is often better if you use fewer kinds but more of each.
 
Final size
Consider the ultimate height and width of the plants
 
Plant stepping
Place the tallest plants to the back with the smallest at the front. An island bed would have the tallest in the centre. The odd focal plant that stands out from the rest can be very effective.
 
Seasons
Remember plants look there best at different times of the year. Use a range to give all year interest.
 
 
Foliage plants
Variegated leaf foliage helps to maintain interest throughout the year. 
 
Mulch
In a shrub border it is worth mulching the ground after planting. It will conserve moisture and look good.