|Get it? We're discussing things that are "tack sharp"? Hehe.|
No tripod? Well, if you're lucky, there's either a steady, flat surface you can place your camera on or there's a sturdy wall you can lean against for stability. Make sure your "Image Stabilization" or "Vibration Reduction" feature is turned on.
Still don't have a wall or table to use? Try holding your breath when you press the shutter button (I'm serious! And don't jab it!). Have as much light on the subject so that your shutter speed is at least 1/60th of a second. If necessary, kick up your ISO.
If all else fails, there's "Unsharp Mask" in Photoshop that will help do the trick in post production (but don't rely on post processing!).