I've not had much of a problem with cabbage whites this year, although I have seen them fluttering around the garden. There have been a few patches of eggs on the leaves but I check morning and evening and rub off any that appear. More of an issue are the tiny snails that seem to have taken a liking to the brassica leaves, more of which later.
Keeping them well watered during the recent hot weather was a challenge as the bed is on a slope. I'm slowing creating a raised bed that will level off the surface but for the time being the water flows towards the lower end of the bed. Also, in hot weather the water evaporates and doesn't really get down to the roots where it is needed.
The method I've used over the last few years is the old water wizard and plastic drinks bottle trick. I received the green plastic, conical water wizards many years ago free of charge along with some hanging flower pouches from Thompson and Morgan. You can purchase them separately if you wish (http://www.thompson-morgan.com/garden-supplies/watering-and-irrigation/water-wizard/p9800TM). They are marketed as a means of ensuring that hanging baskets and containers are thoroughly watered. They work just as well, though, in the main part of the garden. Simply cut the bottom off the plastic bottle and screw the top on to the water wizard. Then push the water wizard into the ground and fill the plastic bottle with water. The water then slowly flows through the holes in the wizard into the surrounding earth. A lot of people have found, though, that you don't need to use a water wizard. Just bury the neck of the bottle in the ground.
A bonus of this technique is that there is far less surface water that could encourage slugs and snails onto the vegetables. There have been very few slugs this year and I put this down to having used Nemaslug last year. The main problem is that of snails. Crushed egg shells around the plants don't work and I try and use slug pellets as little as possible, even the iron phosphate ones. So I am currently trying out a combination of beer traps and garlic wash.
The trouble is that I'm not really going to learn which mollusc deterrent works best as I am deploying several at once. I will know, though, if none of them work!
Update: I'm beginning to think that the eggshells do deter the molluscs as I have one group of plants where I have not used any other method and there has been minimal damage to the leaves. It could be, though, that some of the local bird population have been treating the area as a restaurant!