Depends on the variety of plant - some will have been artificially chilled at a certain point, others will have had daylight length extended or reduced via artificial lighting, some will have been grown in artificial warmth. The growers know what conditions trigger flowering for each type of plant and try to duplicate that.

Also, growers have access to chemicals not available to the average home gardener that control the space between leaves, branching, and stimulate flowers, so the bedding in six-packs can look like perfect miniatures of their natural, full-grown selves, rather than the healthy tufts of foliage that they really should be at that size...wheeze, wheeze....Okay, rant's over! :)

And let's not get into what the growers for the show gardens at the likes of Chelsea Flower Show do...

A trick at shows for flowers that are not quite open is to blast them with hot air from a hair dryer. The hotter air temperature causes the flowers to open.

The main trick is the night-day cycle. They use artificial lights and/or blinds to fool the plants into thinking it is the time of year they would normally flower.

Yup and put bulbs into a cold walk-in fridge to simulate winter then into a heated greenhouse... it is one of the reason's the actual flowers last for such a short time. Friends of ours change the actual pots of bulbs daily at somewhere like Chelsea.

A grower once told me you can get lily of the valley to bloom when you want by keeping it refrigerated for while so it thinks its winter and then bring it out into the warm.
Apparently that's what they do for wedding bouquets.

Oops, MG, just read your reply saying much the same thing.

Tugbrethil do you know the names of any of these chemicals used to enhance flowering? thanks

Often found in ads in commercial greenhouse trade magazines. I can look up a few in the magazine my boss gets, and give you a P.M., Wannabe. Be advised, though, that--like many such shortcuts--they will stimulate flowering at the expense of overall plant health. They may leave you with half dead plants after a few weeks

Not something I'd recommend trying Wannabe. If it doesn't work out and is for a specific occasion your really stuck!

Like most gardeners with summer bedding, I nip off the flowers present on planting out, to encourage the plant to grow more foliage and root rather than flowering at such a small size...

That is the staple technique for dealing with the modern bedding market. Other things that help is to make sure that the soil is rich enough to support new growth, with lots of humates and fulvates to help clean the dwarfing chemicals out of the plant's system, and with good pore space for the oxygen necessary for all the metabolic processes.