When shooting portraits a standard zoom is fine but a longer lens will give you more flattering shots and a tripod can be useful if you're using a slower shutter speed due to the amount of light available. Before picking up the flash gun, reach for a reflector to bounce some extra light into your shot and if it's too bright try hanging netting on the window but stay away from colour as you'll get a cast on your image.
Cloudy days, which we seem to have a lot of at the moment, are perfect for portraits as the light's diffused and if you have a spot or window if you're working indoors that's not in the direct path of the sun even better. Before you start, make sure you clean your window, though, as a dirty one can be a full f/stop darker and turn your house lights off.
If you find that the sun is causing shadows to appear under the nose and/or chin of your model use a reflector to add light to the areas in shadow or if you're indoors, move them more into the room, away from the window.
Don't meter from your window when shooting at home as this will have the same effect as snow has on your camera – it will think the scene is brighter that it actually is and your model will therefore be underexposed. However, by simply metering from your model's face you'll have no problems. To give your shot more warmth try the cloud or shade white balance settings.
The eyes need to be sharp and the front of their face needs to be in the light to draw attention to it. Asking your model to simply tilt their head slightly will bring more/less of their face into the light. You can also move your camera around the scene to find a suitable position to shoot from.
Try sitting your model 45 degrees to the direction the light is coming from or how about shooting side on? For more interesting shots try only lighting half of them or have a go at creating shapes from their silhouette.