If you choose representational painting in any capacity, you'll be confronted with constant tests of your skill. My most recent painting pushed me to develop my background skills. We have a pink rose bush outside our house that I've always marveled at and I knew I wanted to use in a painting. I felt that I hadn't had the skill level to use it so I always shied away from it.
I came to the conclusion that my skill level wouldn't increase unless I tried to paint it. What I found is that you can't paint every single rose and leaf. The key is to actually paint the idea of the flower. This sounds strange, but I'll explain.
My best friend said that my most recent painting looks like a photograph, which made me think of Bouguereau. I recently saw an amazing Bouguereau which I learned a valuable lesson from. Great artists go beyond nature. Most people think that Bouguereau was a photorealist painter, when in fact, I considered him a Photo-Idealist or a Romantic Realist. I don't hide the fact that I'm incredibly influenced by Bouguereau in terms of technique, but also in terms of his philosophy of painting.
I've had my paintings mistaken for photographs before, but my paintings are really an idealized synthesis of nature. I actually don't like photorealist painting because I don't believe it serves a higher poetic function. Obviously, a portrait has to capture a likeness, but is a likeness purely representational? I feel that a likeness is more about feeling.
When you paint a flower, ask yourself if it feels like a flower. Often times, you'll find that you have to paint more than the mere representation of a flower. Sometimes you'll have to paint a lie instead of the real thing, but it'll come out more truthful. Another tip for achieving this is to draw from your imagination before you begin a painting. Your croquis may look like a cartoon at first, but it'll serve as an unfiltered roadmap from which you can employ nature.
For portraits, you'll obviously need some reference material, such as a photo. For this portrait, the dog has passed away, but I had met the dog several times and had a strong feeling for their personality. I honestly used my memory more than anything else to capture the feeling I wanted.
There will never be a one size fits all, linear model for painting, but always try and expand the painting's poetic sense and you'll learn what works.