1. I know you won't like the reply, but: Get a sequencer. Don't ditch Sibelius for what it is, it's excellent, but with a sequencer you'll be able to mix better.
2. I don't think that there IS an actual guide anywhere. It's more about practice, trial and error and forums like this and many other that you can get feedback.
3. A good monitoring system IS crucial. You may not have the money to get decent monitors, or the space (I had my stuff in the living room for two years, before I got a tiny room), but you could get some excellent headphones, which, even if not exactly the same, will help out enormously. Just take your CDs (or your music) with you and be amazed at a store at how they might sound through some GOOD cans.
4. It might not be actually the mixing at fault but programming skills and knowing your library well enough. You have Gold, or Gold xp pro? Do you use CC11, do you use CC1 (modulation), do you use the crossfade patches, do you layer different articulations of strings to get a richer sound and a more interesting dynamic curve? Do you mind if you end up using... 24 French horns, because the 6 french horns patch sound better, even if in real life this would never have happened? Do you bounce, if you don't have enough RAM and mix in audio? Etc...
5. Good sequencing software.
first of all:
REAPER | About . Shareware, no limit. try it, it's free and legally so!
For PC, Cubase (which has an entry version Cubase 4 Studio) and Sonar, which again has an entry version. As a matter of fact I believe that Sonar also has a trial version out. Get it.
For Mac, Logic. Logic 8 comes bundled with some amazing stuff (check with Marius for more).
Pro tools. Yes... it takes a lot, and midi STILL is not up to par with the rest, but it rocks your audio world otherwise (maybe because *normally* you also need the relavent hardware to run it!

)
6. With most (if not all) EW stuff, inclduding Gold you don't need:
* panning
* extra reverb
* EQ
* tempering with the sounds.
If you work on other types of music, yes you need all the above, but just with EWQL Gold (xp pro, or not) you just need what comes out of the box to sound great ultimately. EW products are panned and are recorded in a hall, with the reverb there (release trails it's called). EQ is rarely needed in such music, maybe only in a mastering level, but that's the end of the line.
Hope it helps