The simple answer honestly is just upgrade your sound libraries, or get someone with better samples to render the part you need. With acoustic instruments it's kinda hard to make better sounds out of something that doesn't sound realistic in the first place, basically you can't polish mud.
Though in general for making something sound more realistic you want to be very careful with the velocity of the various notes, notice how when someone plays an actual instrument they don't hit all the notes the same way, some notes are softer, others more intense, etc. etc. You have to think how would a player play the line and adjust the velocity accordingly. Some slight velocity randomization (if the plugin supports it) can always help too, but just a little.
http://the-filmmusic-group.deviantart.c ... -224143947These have to be some of the best free orchestral soundfonts available, try looking to see if what you're looking for is in there.
If you're okay with getting some priced samples I've heard a lot of good things about this sample library for kontakt (thing is you need the paid version of kontakt to use it.... so that's some even more money you have to dish out):
http://www.soundiron.com/instruments/pi ... nal-piano/As for brass honestly I'm not sure any good sources other then EastWest, but they're a bit pricey (really pricey):
http://www.soundsonline.com/Hollywood-Brass