When I take a photo that I want to manipulate, I have a vague idea of what I want to do and sometimes when I try blending it with a different background it works perfectly the first time but other times it doesn't. After having done many, many manipulations I now have a better feeling for what might work and what might not work.
However, I still don't always get it right. If something isn't working put it away for another day and try something else. My best results come from pictures that I am inspired by. It always astonishes me what I produce if I am in the mood and excited about what I am doing.
Also, even if you think you have the perfect picture don't stop if you are enjoying yourself. Sometimes I create a picture that I think is great then I try something more and something more and in the end I think my original picture was quite bland compared with what I produced in the end. Often I will 'make a mistake' and switch off a layer to discover that the top layer blends even better with the layer below. If you are thinking of selling your art think of trying different colours to suit different tastes - you will also find some surprising and lovely results quite by accident.
Experiment and experiment more. We have just renovated by husband's study and the main colours are a dark magenta red with some turquoise. He loves dogs so I wanted to create a picture that would fit in well with an office but still be colourful. Here are 8 pictures I produced from one main picture with a variety of secondary backgrounds and these are only half of them: