Simply put, when working with MIDI instruments, control your use of note velocities and have slight variety in your velocity range. You can stop reading here if you've got the strength to keep things under 70 for the majority of a song, but I still recommend at least peeking at the example.
MIDI velocities, or volume + tonal differences associated with common volume levels, have a range of 1-127, 1 being the quietest 127 being so strong, it should and will ruin your songs. The average producer and under-learned composer do not know this. So often, I see people create music by recording a MIDI instrument, jacking the velocities all to 100 (which, by the way is still a loud forte to fortissimo) or to 127 (even worse). This is bad. If you're doing this, stop.
That's a fantastic way to destroy your tone, especially if you are working with high end sample libraries. It's way better to write with your ranges between 30 and 60 for average volumes rather than between 90 and 127. Then, if you have a solo instrument or want a slightly more aggressive tone, sure 80 is OK, and look at that. You actually had room to go up! Even if you are writing epic Hollywood-style trailer music, there's no need to beat the crap out of your 100% of your instruments for 5 straight minutes. And please, don't make every velocity exactly the same. Give your work the expression you already have in your head!
Not only will you achieve better and way more realistic tone by following this advice, but you will also be forced and able to use layering with other instrument types/sections to achieve more volume. This goes for pop recordings that use MIDI instruments just as much as it does orchestral works.
In case you need a little encouragement to kick your butt into gear and start using MIDI correctly, here's an example of a song I'm working on right now:
This is an oboe from the Vienna Instruments collection in Logic Pro. It is my solo instrument for the first main part of the song, yet that shade of green is a velocity of around 55—and I haven't even set a wide range of dynamics or articulations yet!
And this is my cello section. It starts at 32 and builds up to 76 for the "big part." However, the real jump in volume comes from brass and woodwind doublings. So let's listen to a fully unmixed demo.
That's right! Other than a little bit of batch reverb over each of the three instrument sections and the faders being set to about -15db for now (slightly higher for the oboes and double bass), this excerpt is completely unmixed. Because my velocities are set right, I'm going to have a really easy time creating dynamics, additional articulations, and the final mixdown.