Although i am of the belief that flowers look best in their natural habitat, there are times when nothing looks quite so beautiful than a vase/jar/teapot/glass full of summer flowers. I tend to only choose to take flowers from plants of which we either have a lot, or have become damaged in some way. A windfallen rose, for instance, in my eyes, holds just as much beauty as a completely perfect specimen. I also try to make assets of plants not traditionally used as decoration i.e. dandelions (we all have them, might as well make the best of their lemon-yellow flowers) and montbretia (which is flippin' everywhere). Dandelion leaves are also good in salads, and for pets if you have rabbits or guinea pigs. Just make sure you choose specimens that aren't likely to be used as the local dog toilet!
Some of the prettiest flowers have to be sweet peas, and as they smell fantastic as well, they are perfect for a beautiful, blousy arrangement. In my vase are some snapdragons, montbretia (argh!) and sweet peas.
Next year we're set to have a bed just for the annuals, our 'show' part of the garden, which will look splendid. They'll also make for some wonderful cut flowers, too :-)