The trick to long-lasting glamour is to stay restrained. The container is as important as its contents: long-lasting, deep, sturdy plastic is best for retaining moisture. Ecopots has a good one called Bruges (£18; shop.ashortwalk.com). If you want terracotta, make sure you plant within an inner plastic box or line the existing one with recycled plastic sheeting. Trays beneath your box catch the excess water, saving your sills and reducing moisture loss.
The most effective window boxes are usually comprised of one plant, cascading voluptuously. Pelargoniums are perfect for this, particularly varieties with scented leaves, and have the benefit of being hugely forgiving of lazy watering. Even so, whatever you choose, keep watering and feeding it regularly for best results.