Although the ideas of this paragraph apply to all practice sessions, this is the most appropriate place to bring up this subject: how musically you practice and how loud you practice, especially the more exciting loud, passages. Many students hate to practice when others are around to listen. Others are of the opinion that piano practice is necessarily unpleasant and punishing to the ear. These are symptoms of a common misconception. If you are using the correct practice methods and making terrific progress, and quickly transitioning to making music, there is nothing unpleasant about piano practice sessions.
A common mistake is to practice loud passages loud all the time when learning a new composition. There is no need to play a loud passage loud until you can play it satisfactorily; loudness is added later. By separating the loudness from the technique, you acquire technique faster and with less fatigue, and reduce stress. Once the technique is attained with full relaxation, you can add much more power than you could before, without picking up bad habits. Students who have not completely eliminated stress often practice everything too loud. Thus practicing too loud is frequently an indication of some underlying problem. Of course, there are students who never play sufficiently firmly to produce a solid, clear, tone, and these have the opposite problem of having to firm up their playing. Both problems, either playing too loudly or too softly, can be helped by practicing the gravity drop. The best criterion that you are practicing correctly is the reaction of others -- if your practice sounds good to them, or at least it doesn't bother them, then you are probably doing it right.
The benefits of practicing musically all the time becomes obvious when it comes time to perform. Looking at this another way, practicing without regard to musicality will certainly make switching to "performance mode" an impossible task. What does it mean to play musically? This question can only be answered definitively by application of the myriad micro-rules that apply to specific passages of specific compositions. If you have incorporated all of the musical notations and markings into your music, you have built a sound foundation. Then there are the useful general rules:
(i) carefully connect each bar to the next bar (or measure, or phrase). These bars/measures do not stand alone; one logically flows into the other and they all support each other. They are connected rhythmically as well as conceptually. You might think that this point is trivially obvious; however, when you do this consciously, you might be surprised by the improvement in your music.
(ii) there must always be a conversation between the RH and LH. They don't play independently. And they won't talk to each other automatically just because you timed them perfectly. You must consciously create a conversation between the two hands, or voices, in the music.
(iii) "cresc." means that most of the passage should be played softly; only the last few notes are loud, which means that it is important to start softly. Similarly, for other indications of this nature (rit., accel., dim., etc); make sure that you have reserved space for the action to take place and don't start the action immediately, wait until the last moment.
(iv) strive more for accuracy than expressive rubato; rubato is often too easy, incorrect, and not in tune with the audience. This is the time to use the metronome to check your timing and rhythm.
(v) when in doubt, start and end each musical phrase softly, with the louder notes near the middle. It is usually incorrect to have loud notes at the beginning.
Musical playing feeds on itself. Thus a good performance begets a better performance. This also applies to practicing. Musicality also has no limit -- you can improve no matter where you are on the scale of musicality. The terrifying part of this is the flip side. If you do not pay attention, you can develop non-musical playing habits that can keep on destroying your musicality. This is why it is so important to focus on musicality and not only on technique; it can make the difference between whether you can perform or not. The above points [(i) to (v)] are not trivial. If you focus on them, several things will automatically happen. Firstly, you will be so preoccupied by trying to get them all to come out right that you will have less opportunity to make non-musical mistakes. Secondly, they will set your playing on a sound basis from which to improve. As you hear the music coming out correctly, it will become much easier to further enhance the music than if you had started it wrong.