Sometimes, sudden invitations demand sudden attendance. You don't have the time, to let alone stitch, but even buy a dress. For reasons such, always keep a few plain dresses in your wardrobe. Any plain color, in any pattern. All you have to do then, is add different accessories to it, each time you decide to wear it. This is not only cheap, but an almost free option. For example, casual beach wedding dresses can be accessorized with beautiful necklaces (maybe of cheap pearls), or even just a simple hat. Remember, your accessories need to enhance the look; choose wisely.