You can find inexpensive outsize bags from packaging shops. Add a base layer to each pot before adding compost and use them as filler for tall pots where the plants will only be getting their roots into the top few inches of compost.
Talking of which, there is no need to buy John Innes soil-based compost: a less dense, peat-free multipurpose compost will do fine for annual bedding, but don't let it dry out. The pros mulch their plantings with pale shingle, slate pieces or, on larger plants, small pebbles: use to give each pot a good-looking finish that will also help retain moisture and avoid splashback from wet compost.
The key to a great display is to group pots together so they look like part of a border, each plant complementing the other. Raise some pots on bricks or even upturned terracotta pots to create different levels and make things a whole lot more interesting.
That way you don't need to dream up artful compositions of multiple plantings, or worry about how much space in the compost each plant will need, but you can keep to one plant or group of plants per pot, moving the containers around to get the best combination of colours and shapes.
Thus a scarlet geranium in a terracotta pot might be highlighted by a feathery mass of lavender-blue felicia flowers or be nudged by a trio of mahogany Aeonium Schwartzkopf, like miniature dark-foliaged palm trees; a bowl of caramel-coloured Serenity Bronze osteospermum could sit companionably alongside flowery frills of Black Cherry calibrachoa.
Keeping to the one plant type, one pot principle also means that if one planting should fail prematurely, you can simply remove the whole pot before replacing.
Be wary of teaming deep, dark colours together: they're very stylish, but on their own, you could be looking at a big black hole, especially in bright light. Temper dark leaves and bedding with plenty of light contrast: peach verbena sprays or orange diascia surrounding a chocolate-leaved dahlia; sugar-pink petunias or lemon nemesia providing contrast in form and colour to the purple spires of Salvia Caradonna.
Burgundy-leaved banana plants will look totally tropical when fanfared by scarlet begonias and the bold orange and hot pink blooms of New Guinea impatiens. For a fresh, city-smart look, there is nothing to equal the navy-centred flowers of sparkling white osteospermum, but unless you wish to fly the flag, avoid mixing red, white and blue together.
Bring in decorative edibles. Cherry tomatoes, oak-leaved lettuce, tagetes marigold and leafy basil will grow beautifully in a sunny windowbox; strawberries in a hanging basket won't get decimated by slugs, and the fruits will dangle prettily over the sides, within picking distance. A galvanised builder's bucket flatters a lilac-flowered, purple-fruited aubergine; a trio of inexpensive terracotta pots on a sunny balcony provide a showcase for a collection of scarlet chilli peppers.
If you want to ensure a summer-long display, you will need to dose flowers and fruiting veg with dilute tomato feed, foliage with dilute seaweed feed. You will also need to snip off faded flowers to keep more coming, and with flowers such as calibrachoa and petunias, you will need to deadhead on a daily basis: super-sharp snips or embroidery scissors are the container gardener's secret weapon.