This should be adopted into the garden sense 'It's the soil, stupid'. I know, I know, people often tell me it's a boring subject but so essential.
As you go to purchase or acquire plants, find out about the plant's natural environments and mimic these e.g. I will be planting up a sloped bed close to the house soon and each alpine plant will be carefully planted with plenty of grit and John Innes No.1
I have some strawberry beds to open and prepare. In this case there will be plenty of good home-made compost dug in to the soil to sustain the plants over the next three years.
As for my sweet pea plants I am going to plant. Although they can 'fix' their own nitrogen from the soil, dig in plenty of good quality compost and add in some fish, blood and bone meal.
So, where you are getting new plants, find out about them - talk to friends, read books, look up the internet. Know your plants, know what the soil needs and go for it.
This is the real secret to success.