Hard rules make life easier.
"I don't drink soda."
"I don't go online until I write 1,000 words."
"I turn the computer off at 11PM."
"I don't watch television."
"I'm not playing video games until I have $50,000 in cash in the bank."
"I only work on my main computer, and only surf the net on my iPad."
The problem with "soft rules" like "eating better" or "be more productive" or "waste less time" are that they constantly sap your mental energy.
Soft rules lead to a constant negotiation between your higher-thinking-self and your more base-animal-brained-self, the part of you that wants to run towards pleasure and flee from pain.
It's also good for managing others' expectations. "I don't eat simple carbohydrates, period, ever" is a very easy rule to follow and explain to others. "I'll, umm, have just one slice of pizza I guess" is a much harder to explain to others, for others to understand, and so on.
If you really want to make change in an area, set a very hard and objective rule. Make it indisputable and require no thinking to follow. I don't go to Hacker News, Reddit, look at sports scores, or anything like that on my main work Mac. I'll do so on a second PC I own, or on any other computer like in the airport lounge. This is incredibly easy to follow, so it means that being at my work computer means I know I'm not supposed to be surfing the net.
It's very easy to eliminate a class of food or activity entirely, whereas moderating its use takes much more thinking and willpower. Want to make a change, for real? Set a hard, objective rule. They make life easier.