Master Turbid says, every unit of work you do should consist of less than 20 lines of code and it should be done in less than 20 minutes. You may need to think first how to achieve it. This thinking may take more than 20 minutes, and taking notes of it may take more than 20 lines. Then your unit of work is thinking and that's what should be under 20 lines and 20 minutes. (You may want to change the 20 lines according to the expressiveness of your programming language.)
You probably have a gut feeling in most cases when you are about to start a piece of work that's bound to take more time/code. Like having a quick meeting with five people on some strategic issues; it'll probably take longer unless they are focused and disciplined. Writing a wiki engine from scratch will probably take more than 20 lines; unless you are pretty skilled or you have a wiki library at hand.
In some cases you will say to yourself, "It can be done in a few lines of code, well, with some luck". Or you say to yourself, "It must be easy and I don't want to spend more time with it anyway". Or you even say to yourself, "Let's just put together something quickly and then see what comes next". Resist the temptation. Most flaws in software stem from this trigger-happy approach. If you don't exactly understand what you are doing, if the concepts in your head are not clear enough, bad things are bound to happen. You will get stuck or you will not even notice that you get stuck and cover it with some hazy code. And the sad truth about most programmers is that we can't understand large concepts clearly. Those 20 lines of code is the size that fits into our head.
If you follow this rule, you'll live a happy life. Most of your 20 minute plans will come true, you will accomplish what you aim at. Your code will be built on many successful mini-projects.
Once you mastered this rule, there is still room for improvement. You can just sit and imagine the 20 lines before touching the keyboard. Imagine the exact location of the lines: into which file and which function will they go. If you have to modify existing code, what lines have to be changed and in what way? Suppose that you don't have a keyboard, all you can do is tell a non-programmer over the phone what to do.
20 lines will seem incredibly long at first. It's also incredibly difficult to imagine code changes without looking at the code. Especially if you are spoiled by over-intelligent IDEs that spare you the burden of knowing your code intimately.
Master Turbid smiles and says, there is nothing new in this idea. Related concepts were already expressed by other masters, such as Master Pomodoro and Master Arc.