I do a similar thing. I take a midi of the song, and make it sound the best i can. Then take any individual tracks that sound badly, like an acoustic guitar or something and add real tracks to replace them, and then play any one that that does not fix and keep working them until i am happy that it doesn't sound midiish.

Sometimes midi tracks just need a good shake down and a better sample patch. Open them in the piano roll and look at where the notes start and end, the mechanical sound can be tweaked out by dragging them longer and shorter so that the over lap, don't start and stop on a dime, and seem human.

Vocal oohs and aahs are a good example. then all end at the same time, have chopped of edges, drag one to it over laps another and when there is three or four notes together, make then all end at slightly different times. Lastly use a good sounding sample. I use a vocal collection in sampletank 2 One patch is called beach boy, it is three blended voices that are tight and sweet.

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