'Rembrandt', 'Clamshell', 'Backlight', 'Rim-lighting' – these are all tried and trusted portrait lighting setups, each of which gives its own distinctive look. If you practice getting these set up, it leaves you free to concentrate on your subject.
If you're photographic head and shoulders portraits with a speedlight, the secret is to move it off-camera and use a softbox or other flash modifier to give a softer light. This will immediately give a more rounded, flattering light for faces.
Photographing couples is not as easy as it sounds. The answer is not to show them side by side in a perfectly symmetrical arrangement, but to find off-center poses or activities where your couples are at a different height.
Natural light is softer and easier to work with than flash, and by changing the distance between your model and the window, you can change the intensity and softness of the light.
Work out what lighting you're going to use, the clothing your model is going to wear, and work out a list of poses you want to try. The model may have some ideas too. The more you can plan ahead, the more stress-free the session.
You don't need a high-powered professional flash here, just your camera's built-in flash or an external speedlight. The idea is to just brighten the shadows, not to overpower the existing ambient light.
Most camera and flash makers now offer remote wireless flash control for one or more flashguns, and this is a great way to experiment with different lighting angles and setups – and there won't be any cables to trip over.
It's an old technique but a great one. Instead of pointing the flash directly at your subject, you turn it to 'bounce' it off a nearby wall, ceiling or other reflective surface. This is why more advanced speedlights have tilt and swivel heads.
It's a classic cinema technique, using gobos (something that 'goes between' the light and the subject) to create the lighting pattern of window frames or Venetian blinds, for example, and you can use the same technique for portrait photography too.
You can get beautiful baby and newborn portraits using soft lighting and a prime lens with a very fast maximum aperture. The shallow depth of field emphasises the softness of the subject.