Despite having beautiful flowers, the heads of my giant dahlias are all floppy - the weight of the heads is too much for the stem.
What method do the professionals (and not so professional) use to hold them highand make them look good?
Feed them with a high potash feed from when the buds form.
Thats always assuming the plant in general is supported with stakes.
A photo would give me a better insight into your problem.
To put it simply this can be a problem with many Dahlias between 2" and 12" in diameter.
The cultivars in question are Grand Prix, White Alvas and Almand's Supreme.
I don't have pics at the moment, but when the rains stops I might pop out with the pinhole brownie.
I don't know the variety Grand Prix, but White Alvas and Almand's Supreme are giant decs.
The key to good 'foot stems' (the stalk) is early on in the season when you de-branch down to three flowers (max) and constant dis-budding.
As I mentioned before high potash feeds from bud formation.
I have written a bit here on Dahlia culture that might help; http://tinyurl.com/2lbqtn