Use point focus if its available on your camera. Get close as possible to your subject and if needed use the flash.
The shot below was shot with a Tamron 70-200 @200mm f18 or so...natural light, no flash. Stopping down (using a higher f#) can work well or you can open wide open... but I would suggest playing around with a depth of field calculator so you can see what you are getting in that regard.
Most importantly though pay attention to your composition. When you have lights and darks so contrasting each other even in the background it makes the photo kind of weird. Shoot from a different angle if needed... get on a step ladder and shoot down or up or something...but you should always pay attention to your background and its proximity to you and the colors presented.
Everything in front of your lens makes your photo. Darks on darks sometimes makes a weird photo. You want to make something stand out. I read a good chapter in a book that mentioned 'composing with color'...it will all add up in the end.