If the light coming through the window is strong, position a sheet of white card on the opposite side of the subject to the window. Light will be reflected back into the shadow side of the subject. Altering the angle and distance of this card gives you considerable control over lighting effects.
Elephant Hawkshead Moth, You Connect member Dean Powell, Canon PowerShot G5
A sheet of white card can also be used outdoors when photographing small subjects. It is surprising the effect this small reflector can have. It does not need a stand – just hold it near to the subject (but outside the lens field-of-view) as you take the picture.
If you need to use the on-board flash try to enclose your subject with a white card on all sides and above. The flash reaching the subject will also be reflected from the card, softening and diffusing the light.
PowerShot G series and some SX series cameras offer the potential to use the Speedlite range of flashguns. These allow you to use the flash unit further from the subject than the camera to help the exposure, and you can angle the flash head so that the illumination reaches the subject. It is also possible to use the dedicated Macro Ring Lite MR-14EX and Macro Twin Lite MT-24EX flashes with the G-series cameras with an adapter.