Ooh this is the first thing (or one of the first things) to lead me to MN. I use Daz or Persil (prefer the smell of Daz though), Napisan and warm or hot water depending on what the fabric is. Even if I do a hot wash every other time I do a load of whites it keeps things from doing that progressive greying that used to drive me mad.
The smell of most powders is too strong for me so I usually use half powder and half Ecover liquid (always with a dash of Napisan), but as long as some bio powder is used it keeps things white.
Anything really grey or dirty gets a soak in an oxy powder (own brand seems to work fine) and hot water before it goes in the wash. Oxy added to the wash seems to help if I run out of Napisan.
For items that aren't 100% white (eg white tea towels with a coloured stripe) I try to keep them to a separate wash if I can, or I put in some colour catchers in a lingerie bag. Off-whites, cream, beige etc go in a separate wash.
Usually someone comes on these threads to say that the enzymes in bio powders are meant to work better in cold water but I can say without a doubt that a hot wash is more whitening. I blame my eco-devotion to cold washes and Ecover (plus a bad experience with bio powder back in the day) for everything going grey over time.
DH has eczema and I worried that the bio powder wouldn't suit him but over time his stuff has crept into a bio wash and he's never reacted to it, but an extra rinse is also an option.