Having a lucid dream requires, by definition, knowing that you are
dreaming. If there were no differences between dreaming and waking
life, there would be no way to know that you were experiencing one
state and not the other. Fortunately, there are characteristic
differences between the two states that allow you to know whether
or not you are dreaming. In other words, there are features that
make dreams “dreamlike.” Learning to recognize these distinctive
features, typically termed “dreamsigns,” is one of the most basic
and powerful methods of inducing lucid dreams.
Examples of dreamsigns include: miraculous flight, changing writing, malfunctioning devices, and meeting deceased people. By studying your dreams you can become familiar with your own personal dreamsigns and set your mind to recognize them and become lucid in future dreams.
The cultivation of dreamsign awareness as a method for learning lucid dreaming is described in full detail in Exploring the World of Lucid Dreaming (EWLD), and A Course in Lucid Dreaming. The Course also provides exercises for noticing dreamsigns while you are awake, so that the skill carries over into your dreams. This exercise also applies to lucid dream induction devices, which give sensory cues—special, artificially-produced dreamsigns—while you are dreaming. To succeed at recognizing these cues in dreams, you need to practice looking for them and recognizing them while you are awake.
Most dreams you recall will contain at least one, but more likely several dreamsigns. Until you have developed at least a moderate degree of lucidity, you will almost never recognize these dream oddities for what they are, and this leads to a pitfall which can block progress until it is understood and corrected: the mistake (common among novice lucid dreamers) is to focus on how uncritical their minds are during dreaming, using each missed dreamsign as another example proving that they “never recognize dreamsigns”. This is a mistake! If you do that, you use missed dreamsigns to learn that you're too unreflective, stupid, or whatever to become lucid. This isn't what you want to learn, is it?
What you want to learn is how to recognize when you're dreaming by getting to know your dreamsigns. Thus you should make sure that you reflect on which parts of your dream could have told you that you were dreaming, and resolve that the next time something like that dreamsign reoccurs, you remember that you are dreaming! So, if you wake from a dream in which you fail to notice that the friend you were talking to has been dead for years, you firmly resolve that if you ever see that person again you will realize that you're dreaming. Furthermore, resolve that you will see your friend again, and that the next time you do, you will become lucid.
Missed dreamsigns are stepping stones across the river of forgetfulness to lucidity. But only if you use them as such; only if you decide with complete conviction that you won't get fooled again. Of course, you will. To err is human, but why not learn to err less and less?