From "The Essential Guide to Close-up Photography":
Flowers are a favorite close-up subject. Not only do they create vibrant and interesting images, but they are also easily accessible to all. Whether you're photographing wild flowers like foxgloves, snowdrops and poppies, or cultivated species like lilies, orchids and tulips, the approach and technique are generally the same.
Weather is a key consideration. Still days are best as flowers will sway in windy weather, making it nearly impossible to focus and compose images precisely. If you have no choice but to shoot in windy conditions, try steadying flowers using a clamp or windbreak. Overcast light is well-suited to flower photography, as the lower levels of contrast will allow you to faithfully capture color and fine detail. A polarizing filter can remove glare from reflective petals or foliage, helping restore natural color saturation. Early morning and late evening light is warm and attractive and will complement flowers. Backlighting your subject will create atmospheric results, highlighting the translucency of petals and leaves and creating attractive rim lights around your subject. If you have no choice but to shoot in the middle part of the day -- when the overhead position of the sun results in quite harsh, high-contrast light -- keep a reflector close to hand. This will enable you to "fill" dark shadow areas by bouncing light onto your subject.
When photographing flowers, a shallow depth-of-field will often create the most artistic-looking results. Opt for your lens's maximum aperture -- normally f2.8 or f4 -- and focus on a key area of interest or detail; for example, the flower's stamens or the tip of a petal. With depth-of-field being so shallow, everything but your focal point will drift attractively out of focus, directing the eye to your chosen point.
Pay close attention to the subject's background. Simplicity is key. Normally, a nicely diffused, out-of-focus backdrop -- free of mess, clutter or distracting elements -- is best. To create a flattering background, only photograph flowers that you can easily isolate from their surroundings. Look for flowers taller than the ones around it, or which are a good distance from the background. Remove distracting grasses or twigs using scissors or flatten by hand. If possible, position yourself so that other out-of-focus flowers form the background, creating a pleasing wash of color.
Of course, you don't have to photograph flowers outdoors. You can bring flowers in from your garden, or pop to a local florist for a handful of blooms. Shooting indoors allows you more control over lighting and background color. You can position sheets of colored card or fabric behind your set-up to form a simple contrasting background. Window light is a great form of illumination, so place your flower on a table where it will be bathed in natural light. The light can be diffused if necessary by hanging muslin over the window. By shooting indoors, you can capture great floral close-ups whatever the weather.
Finally, remember to only photograph pristine subjects -- in close-up, even the smallest imperfection will be highlighted.