Also note that string parts sound most effective if they are able to sustain and fade out naturally when you want the part to end, so allow the time and space to allow this to happen. Staying with background instruments for the moment, most modern synths come with a plentiful supply of pad voices of different kinds. Be careful which you choose if your main background sounds are pads. The sound chosen needs to be very interesting because there will probably be little going on in the pad part rhythmically. Another idea to enliven a static pad is to arpeggiate whatever it's playing, possibly adding timed delays, if that fits with the feel of your track.
Choruses are very important.
Indeed, some arrangers recommend working on arranging the chorus before you do anything else, because the other parts of the song should proceed from it. The effect is unsettling, adding tension and twisting the tune to show new aspects of itself, and once more helps to maintain listener interest. After a certain number of repetitions, even the most catchy of choruses can begin to pale. Well, a change is as good as a rest. This c reates a feeling of instability.