This is a dangerous question, so let me try to phrase it correctly. Premature optimization is the root of all evil, but if you know you need it, there is a basic set of rules that should be considered. This set is what I'm wondering about.

For instance, imagine you got a list of a few thousand items. How do you look up an item with a specific, unique ID? Of course, you simply use a Dictionary to map the ID to the item.

And if you know that there is a setting stored in a database that is required all the time, you simply cache it instead of issuing a database request hundred times a second.

I guess there are a few even more basic ideas.

I am specifically not looking for "don't do it, for experts: don't do it yet" or "use a profiler" answers, but for really simple, general hints. If you feel this is an argumentative question, you probably misunderstood my intention.