Mining 101

Mining succesfully is the key factor to any victory. The more resources you have, the more troops you can generate, the more chance of winning you have. There are three key factors to succesful mining/gathering

1. Have enough workers (SCVs/Mules/Probes/Drones)
2. Keep an eye on resources left in fields
3. Protect against drops

Having enough workers

If you don't have enough workers, your resource patches will sit idle and you will "lose" money per second. They key here is to make sure that on every mineral patch and gas geyser that once a worker leaves the patch to return with resources, another takes its place. In Starcraft 1 this meant somewhere between 1½ to 2 times as many workers as mineral patches, and 3 workers per geyser if the main building was as close as possible to both minerals and geyser.

An eye on resources left

In order to maximize worker efficience, it is important to keep an eye on when your resource patches will run out, and begin preparing to send workers to other bases once they can't harvest from their patch anymore. Although you should always be looking for new places to set up camp, the effort to defend them can take precious time and resources from the general war effort. By noticing when your resources will be depleted, you can take steps to set up camp in a new spot.

You should begin moving your workers somewhere else a short time before, or once mineral patches go empty, otherwise the workers will sit waiting for their turn at the patch, and you could have four workers mining and eight waiting (or worse), and those eight should then be moved elsewhere.

Protect against drops

Drops (airlifting attackers into the worker camp) is one of the greatest threats to your workers, and you should always take steps to minimize the risk of oyur oppenent pulling of a succesful drop, by having defensive measures in place to protect your workers. Destruction of workers is one of the greatest threat to your economy, as a destroyed worker will have to be replaced by a new one, which costs minerals, plus that worker can not mine (it's dead) while a new one is being built. This site posted a very thorough calculation of the true cost of losing workers in Starcraft 1. The results are quite shocking: Losing 10 workers comes at a cost of 1160 minerals. That can be a devastating blow to a players economy, so:

Protect your workers

This means either ensure that you have sufficient air defense to take down dropships before reaching your base. However, against a skilled player, this can be nearly impossible, so also be sure to have ground defense to take out any dropped units. The ground defense MUST be able to reach as far as any dropped units that can reach your workers.