ISO 800, f/2.8, 1/500sec
The key to the equation is to keep the shutter speed high enough to override the camera/hand shake factor. Because macro is sooooo close up, camera/hand shake is amplified. In each of the following pictures, I bumped my ISO just high enough to buy me a fast enough shutter speed. All pictures were taken with my 100mm macro lens. All images were taken hand held. All images were shot "wide open" (except as noted) at f/2.8 , which also helps to buy me extra shutter speed.
ISO 640, 1/640sec
ISO 640, 1/320 sec
ISO 640, 1/640sec
In the picture below, I wanted a less shallow depth of focus. At the very least, I wanted the entire center of the flower to be in focus (thus, my smaller f/stop of 4.5). Oftentimes macro photography will require small f/stops like f/8, f/11, even f/16. Those small f/stops will NOT work with my technique, as they require a slower shutter speed to let in enough light to compensate for the smaller aperture. But for the picture directly below, I had enough light to use a slightly smaller f/stop while still maintaining a fast enough shutter speed to handhold the camera.
ISO 500, f/4.5, 1/400sec
ISO 400, 1/640sec
ISO 800, 1/250 sec (this is about as slow a shutter speed as I can handhold effectively)
There is an added benefit to my "lazy" technique for outdoor macro photography. If there is any wind at all, a tripod + slow shutter speed combo is going to get you nothing but a blurry-blowing-in-the-breeze flower picture. A fast shutter speed, however, will override not only your shaky-hand-held-camera effect, but also will "freeze" the movement of the flower. Both pictures below were taken, handheld, in breezy conditions...
ISO 400, 1/1000 (fast!)
ISO 640, 1/400sec
FYI, all these flowers, except the last two outdoor ones, are all store-bought bouquets from Trader Joe's. So feel free to go buy yourself some winter cheer and try some macro photography. Oh, and if you don't have a macro lens, experiment with reverse mounting a prime lens you already have ...