Greetz NG
I've been wanting to write this for a long time, but I've been too lazy to gather my thoughts into an organized form.
There are a LOT of things to consider in orchestration and I'm going to share with you what I know, to try to help better your symphonic pieces.
1 - BREATH. Your flutist is not a god. Any wind instruments(horns, reeds, choir) need phrasing that allows a player to breath. Add commas(spaces for breath) to the middle or end of your phrases. An easy way to do that is to have your phrase end half way through the last measure of that particular phrase. Once you start thinking about it, you start to realize how unnatural it sounds for a wind instrument to just play infinitely.
2 - NUMBERS. You do not have a thousand person orchestra. Be wary of stacking up too much harmonies with something like a "String Ensemble" patch. These are mostly recordings of, or emulations of a full orchestra playing one note. Every time you stack another note to create a harmony you are doubling the number of players. This becomes a bigger problem with samplers, because a simple triad might turn your piece into a 210 player song and the mix will muddy quite quickly. A solution to this is to use separate string sections that each play one note of the chord you are trying to play. For example you have your violins playing a C, violas playing a G, cellos an Eb, and double basses a C, and you have your a nice rich and grand sounding Cm chord. Also, it's safer to stack these individual string sections harmonies as doubling the players won't be as extreme in terms of numbers.
3 - DIVERSITY. You are not Phil Spector. A wall of sound is il-advised. It is very important to have a diversity of dynamics and articulation to have a believable score. Not just the song as a whole, but each individual line as well. Try to imagine the dynamics of each instrument as a single weave on a basket, they should go over and under according to the placement and emphasis of other instruments. Proper use of dynamics can effectively solve transitional problems, and help to create deeper texture to your piece. Having various articulations can easily liven up a piece. First off, it can help solving the problems of need breath by switching from legato to staccato. Secondly, the role an instrument plays at any given time is mainly defined by its velocity and articulation. A loud legato horns will likely play the main melody, where as a soft staccato one is more likely to be part of the accompanying harmonies in the background, or part of thee rhythm section. A side note to those who fake articulations (for example using a legato string patch for what should be a staccato note), try to realize that intonation is very important in creating a believable fake. Good use of ADSR filter and volume envelopes can really help here.
4 - PLACEMENT. The orchestra you are listening to does not live in your speaker. Placement of instruments is extremely important to the over-all quality of your piece. Proper placement of instruments in a stereo field not only helps space out your mix, but also helps to "worldize" (make it seem more like youre there) the piece. Orchestras have a general seating layout that is pretty universal, with some minor differences in things like solo instrument placement for concertos and percussion placement in pieces with a lot of percussion. It's pretty simple to place an instrument using panning. I suggest, that you reference the picture below, or look up orchestral seating charts if you have a disagreement with this particular one. To add ontop of all of that, you need to put your players all into the same room by use of reverb. Discussing which types/products of reverb are best for this is a whole topic of it's own, but using a very subtle convolution reverb on the master track can effectively solve this problem.
5 - SCREW UPS. Nobody plays perfect, so if you really want to add that final humanization to your piece,it is important to add small variations in rhythm accuracy and velocity. Even if everynote of one line is supposed to be the same velocity, please don't do it. Many DAWs have variation scripts to allow you to easily randomize the settings of velocities and rhythms by a certain small amount. This final touch is key to creating a believable orchestra.
Now if only I could force all the symphonic composers on NG to read this! Blarg! Feel free to add any other tips you've collected. I'd love to hear them. And as always,
Cheers!