build each part in octaves giving them a note like building a chorus in harmony.
the bass and the highest part can follow the same note but switch the in between string chords to harmonize with the rest. do this until you build a strong layered sound.
just get a viola, chello, violin, a flute and play them all together that should sound nice. look at the different instruments used by an orchestra and notice how they are used with eachother and experiment with a MIDI orchestra...
