When working from photographs, the better the reference image, the better the painting. Post-processing applications, like Photoshop, can enhance images to a certain extent, but taking a good photograph to begin with can make a world of difference to your painting. Below are some tips for getting that 'perfect shot'.
Image Resolution
Use the best camera you can get your hands on, preferably a Digital SLR. Modern smartphones take amazing pictures nowadays, but they are still often pixelated or blurred when you zoom in or grainy when you take pictures in low light situations.
If you are familiar with RAW image formats, they are great because they capture more information in the shadows and highlights than JPG images.
Once you have 'the perfect shot', keep the original file safe. It is this file that you want to use for the painting, not the image that has been via social media, copied and pasted into a document or compressed while being emailed. You want to keep your image completely uncompressed.
Good lighting makes a bigger difference to your photography than a good camera. Daylight captures the most natural colours, while the glow of indoor lighting usually creates warm hues. These warm tones are also produced outside during the golden hour (the hour just before sunset). However, you should also consider the intensity of the light. The sun can make for very harsh shadows and sometimes overexposed faces in the middle of the day. Around dawn or dusk is often much easier for softer shadows, but cloudy days can work well too, as clouds diffuse the light. Taking pictures next to a window with the lights switched off can create really nice lighting effects.
Sometimes portraits taken with all of the light coming from one side can lead to very dark, blacked-out shadows. To add some detail to your shadows try holding a reflective surface, such as a piece of white card, on the dark side of the model to reflect a bit of light back onto them. 
Positioning your object or model right in the centre of your photograph can create a real impact, particularly if your image is square or round. However, placing everything in the centre can look very amateur. If in doubt follow the rule of thirds by placing the main focal point approximately a third of the way across and a third of the way up/down the image. 
Diagonals can add movement to your compositions; look for ways to break up your composition with diagonal lines or shapes.
Think carefully about the colour palette you wish to have in your painting. Dull, monochromatic paintings can be very effective, but can also lack energy. However, overly colourful paintings can distract you from the main focal point of the image. A 'splash' of colour can be really effective, but go too far with this and it can look tacky. 
As well as considering what colours are seen in your object or model, consider what colours are in the background. A bright red object in the background, for instance, could distract you from the main focal point. 
Colours are very personal as much comes down to your personal preferences. But do put some thought into it when you are taking your photographs.
Context, meaning and storytelling
Placing your object or model in context can change the way you see it/him/her. Think about how people viewing your painting will interpret it. For example, do the objects or scenery in the image change the way you think about the model? Does they tell you something about who the model is, their personality, their life or their thoughts? These things will always be incredibly subjective, but remember your job is to spark people's curiosity, not spell things out.
Perhaps you want to take your painting to another level of meaning by considering different forms of symbolism? For example, different colours suggest different emotions, different flowers suggest different occasions and different animals suggest different personality types. There are many ways to create visual metaphors in a work of art and some are more obvious than others. This is entirely up to you, but often subtlety is most effective and arousing curiosity can make for interesting talking points.