Open the aperture as wide as possible
Use a higher ISO
Slow the shutter speed if motion blur is acceptable
Add more light with a flash or continuous light
Use a tripod or stabilize the camera
Shoot in RAW
Focus carefully, preferably on a high-contrast area
Use lens stabilization if available
Avoid zooming in too much if it reduces light
Expose for the subject and protect highlights
Use manual mode or aperture priority for better control
Reduce camera shake by using a remote shutter or timer
Keep subjects still when possible
Clean the lens to maximize light transmission
Use a faster lens with a lower f-number
Increase scene brightness by moving closer to the light source