If the soil is very compacted then you could double dig. This involves digging down 2 spades depths and mixing compost into the bottom part. It also depends what type of soil/rock is underneath. The more you can do the better the plants will grow. However, the plants should only be planted as deep as they are in their pots.

its worth leaving the plant in a small depression as it keeps the water round them better while they grow but as volunteer sais dont plant higher than it is in the pot .

If you have particularly dry, free drained soil, I would perhaps mulch around the base of each plant after planting with composted plant material the courser the better to help keep moisture in.