Jonah. I do this quite a lot, with a variety of clean sounds: piezo pickup through a graphic EQ is my favourite for acoustic simulation. Single coils through a clean AC30 or Mesa, plus the EQ, also works well. Like all patches, you'll have to work out the EQ setting which suits your guitar and amp and speakers. For acoustic songs I like a U-shaped graphic.
For the string sounds I always use a GR-33 synth, driven from the piezos. A GR-2 hex pickup also works, but you have to be more careful with the tracking. The GR-33 has a couple of string pads with long rise and decay times which float happily in the background. I use three expression pedals, one for synth volume, the second fades in the same synth sound one octave up. This is often more effective than raising the volume at "poignant" moments. The third pedal controls guitar volume because, if you are playing without a sound man, then you may need to adjust the volume without stopping playing.
One other trick is to use a 4dB mid boost which you can kick in for added punch in the chorus, or if the audience starts singing along.
