An underestimated part of (online) Go is time management. Like the opening, middle game and end game technniques, time management is something that needs exercise. It doesn't come all by itself, or rather, it does in the wrong way. Good time management is a feature of strong players. Have you seen blitz games by 7d amateurs? They usually consume the full overtime period. They also use most of their available overtime periods. Here are a few suggestions for good time management at the beginner and amateur level.
In real life I prefer a game with a good amount of main time? and then a fast paced overtime to conclude. Online I prefer the other way round: a small or no amount of main time, moving quickly into a slower paced overtime. The reason for this is that you never know what happens online and I want to avoid having to quit a game because my opponent has left or was cut off from the internet. In overtime, the game adjourns more quickly. In real life there is no such burden, so there I want to be unconcerned with time for most of the game.
As most of us play most of our games online, let's have a closer look at overtime settings.
I prefer the so called Japanese overtime, with 5 periods of 20 to 30 seconds per move. A corresponding Canadian overtime would be 5 minutes for 10 to 15 moves. Canadian overtime allows you to think longer about crucial moves or sequences, then play out the consequential ones at a faster pace. On the other hand it also requires you to keep track of overtime left. For someone still learning how to manage time, Japanese overtime is easier.
Incidentally, I advise against absolute time. One should not suffer from having to answer moves.
There are two aspects of managing your time: using the time you have and not running out of time. Let's say the game settings are 5 overtime periods 30 seconds per move. How to manage these periods so that you are using the most of it while not running out of it?
The first recommendation is to always use almost one overtime for every move, even if you have a strong idea of what it should be. This will allow you to catch things like atari blindness or other misreadings. It will also enable you to think a couple of moves deep and think of alternatives, improving your tactical ability. Meanwhile, you still remain 5 seconds away from losing an overtime period.
The second recommendation is to consume all but one overtime periods before the game ends. It would be a pity to lose the game with lots of overtime still in the bag. And because you don't know in advance who will win, better be safe than sorry. So how to decide when to consume an overtime period?
Last but not least, your opponent has the same amount of time allocated and unless they treat the game as blitz game, you can use the time they are thinking to think about some alternatives as well. Where would you play in their stead? Where are your vulnerable groups? Will they respond to your move or resist? How will they try to get sente? Etc ...
Time management can be practiced!
Set up a few games with a focus on time management. Try to do what I've recommended and see how it works. Are you able to use your time or do you tend to rush? Can you resist the sloth of letting an overtime pass for no good reason? Are you capable of sacrificing an overtime to a tough calculation or do you keep ending up with too many overtime periods? Can you force yourself to think during the opponent's time?
Practice will bring you up to a level where time management becomes a second nature and you can use all the time you have for concentrating on Go itself.
tapir: Something I learned about time management (still do horribly under pressure) is to spend time thinking before it is too late. I.e. take time to think through crucial decisions, not to recover later. Half of the time allowance is that of your opponent, often some time spent in thinking through things early against someone, who is rushing through the opening, is repaid by getting to use your opponents time later.
Dieter: Ah, a very important point indeed that I failed to mention: use the opponent's time too! Thx tapir. (edit: added this as a separate lemma)