For the recording musician, MIDI is a staple of the process. Very few of us are lucky enough to be able to play all of the parts on our recordings on the actual physical instruments. So, at some point we load up a MIDI instrument to add a needed part to a song.
Some of these MIDI instruments use sampling techniques of live recorded instruments, and some are synthesizers. Whichever the part calls for, keep a few things in mind;
1) When you are choosing a MIDI instrument, there is often more then one choice. Try to choose the instrument based on the sound and how it fits in the song. This will make the part sound better, and make mixing easier!
2) Resist resorting to automation for controlling a note's loudness Most MIDI instruments, especially the sampled ones, have multiple velocity levels. In other words, a different sample was recorded for each note depending on how that note was played. There are often several levels, with different samples for each note. Make use of this to create a more natural sounding part. When musicians play, their instrument sounds different depending on how hard they attack the note. A softly played note will sound different on most instruments from a loudly played note. JUst turning up/down the volume with automation will not sound the same. So, use different velocity levels for the notes for different parts of the song. Even varying the velocity from note to note within a measure will give it a more natural feel. This will go a long way toward making the part sound more like it was played live, and less like a "canned" part.
3) Don't get carried away with notes! MIDI can play a lot of notes at the same time---much more than a human can play. But this should only be done if the song requires it. On drum parts and percussion parts, in particular, keep in mind that a real drummer has only two arms and two legs. If you want the drum part to sound "real", then restrict the part to what a live drummer is capable of playing. We have all heard those drum parts that are too busy, and sound frenetic in the background of the song. Avoid that effect by keeping your parts simplified to what can actually be played by a real musician.
4) Consider recording MIDI instruments as MIDI, rather than as audio. Having the MIDI allows very easy editing, and polishing of the part. It allows totally changing the notes, the key, or the instrument on the fly. Maybe you start out with a piano sound, then later decide a Rhodes sound will suit the song better---a very easy change with MIDI. Simply select Rhodes for the instrument sound. If you recorded the audio of the part, you have to go back and re-record it using the Rhodes sound instead. Will you get the same performance? Maybe. But, if you recorded the MIDI, the same performance is guaranteed!
And MIDI might be a help even before the first track is recorded! What about the "click" track? Typically, click tracks are very bland and monotonous. They use some sort of beeps or clicks to provide the timing for you to follow as you play. But, these sounds are not what we are used to playing to. And, this makes them unnatural for playing along as you record the first tracks of a song. You might end up with a very sterile sounding recording because there was no natural "feel" to the playing.
So, try abandoning the typical click track and replace it with some MIDI drums. You do not have to do any radical drum programming at this point. Just do a basic drum beat with kick, snare, and high-hat---maybe throw in a cymbal crash or two. Use it to establish the initial rhythm and feel of the song. Then repeat it as many measures as is necessary to play through to the end of the song. I think you will find that it is much easier to play along with the drum track then with a click track. And the more comfortable you are while playing, the better your performance will be.
And, as a bonus, if you were planning to use MIDI drums anyway, you have made a good start on setting up the part. Just modify as needed for bridges, fills, and choruses, and you have your drum part!
So, we are leveraging MIDI to not only give our song instruments it needs, but to make them sound as good as possible too. Remember, everything is in service to the song. So create MIDI parts that provide what a good musician would play! Your songs will sound better for it.