That's a mouthful of questions my friend, but I'll have a quick go at it for you.
Space and clarity are two separate realms of discussion and equally important beasts.
Space, sonically speaking, can be achieved in a variety of ways such as cleverly panning certain sounds & instruments, use of reverb and/or delay FX. Reverb can help a sound sit "further back" in the mix, but can also add a sense of cohesion to a track when used in an extremely subtle manner (example: FX Bus with Reverb, all channels are "sent" to bus with very minimal values). Delay can add depth and width to an otherwise static/monophonic sound. It works well for vocals, percussive hits and even synththesizers just to name a few.
The key is knowing when and how to use effects such as reverb and delay in addition to understanding the idea behind panning certain sounds. A studio drum kit, for example, typically uses a single (i.e. mono) mic for the kick, snare and hat. While we would ideally want to keep the kick and snare dead center, that hat can be panned either to the left (from the POV of the drummer) or to the right (from the POV of the listener). So what about the cymbals, toms, etc.? I've seen instances where a single mic is used for the hi & mid toms, a single mic for the bass tom and a mic per cymbal with each element panned either per drummer or listener POV. Take it a step further, a pair of overhead microphones (L/R) can be used to capture the cymbals as well as bleed from the other components of the drum kick. By mixing this stereo overhead channel into the rest of the kit and summing the lot to a bus, you can have yourself a tight sounding kit with a bit of width. The point is, as you're creating your drums think in part like a drummer and in part like an engineer.
Clarity is quite broad. Sometimes a hi shelf EQ can add a bit of sparkle to the high end, othertimes a bit of saturation can beef up an otherwise puny sound, making it stand out more in the mix. There's is simply no quick answer to getting the "pro" sound, as I'm sure many of the pros on this very forum will confess that to this day they're still perfecting their own means to get the most clear sounding mix.
Here are a few tips to help achieve the clean mix you seek:
Low cut everything. Seriously. Check every hi-hat, snare, percussion hit, etc. There's a 99% chance there's some low frequency rumbling shit going on that will do nothing to your mix except eventually muddy it up and devour headroom. Low cut just enough to get rid of it, but not so much where the character of the sound changes.
Check Your Phase. Continuously check your mix in mono. If your main lead sound suddenly disappears or your mix suddenly sounds flat, you've got some phasing issues going on. This can be corrected in a variety of way: using a "mono-izing" plugin (aka removing the M from the sound via M/S), checking the settings of a plugin setting that is too wet or perhaps a synth setting with an abnormally high unison spread). Place lossless versions of some of your favorite tunes in your DAW and sum them to mono to get an idea of what elements are essential to the mix and which ones can afford to be attenuated a bit.
Surgical EQing. When using samples, this is a must. When creating sounds via synths, you won't need as much surgical EQ, but it's always good to atleast check. Surgical EQ is crucial with regards to eliminating rogue resonances that can really fuck up your mix, specifically the mid-high and high end.
De-Mud. Around 300Hz is the mud range. Subtle bell notch cuts can really help out with low frequency sounds (kicks, basses, etc.).
Keep Your Bass Mono. This is where bass layering and/or splitting can come in real handy. You always want your low end/sub frequency content to be dead center or you're asking for trouble down the road. You can still have a bass sound that is "wide," but make sure it's the higher frequencies that are getting the stereo treatment, not the low ones. Massive in particular is one synth that you really need to watch the use of additional voices/unison spread/panning.
Shit in, Shit out. If you've spent more than 20 minutes on a single sound trying to get it to fit in your mix, find another sound. The type of sounds you choose for your song will ultimately determine the quality of the final product. Shitty samples & sounds will always sound like, well, shit–no matter how much processing you apply. Remember: you can't polish a turd.
Trust Your Ears. If your eyes are telling you one thing but your ears are saying another, go with your ears and move on.
Work Fast. Don't spend 30 minutes looking for the perfect snare or scanning through presets/sample libraries. You will find that when your session is over, you've accomplished nothing and have a serious case of ear fatigue. Rather, focus on making a nice 16 bar loops with all of your components (ex. drums, synth(s), bass, etc). Once you're happy with the groove, move on with fleshing out the arrangement.
FX.Sweeps, downers, uplifters, impacts, etc. are great ways to add some "filler" in your tracks to prevent monotony. They can also help with transitioning in your arrangement as well as creating the tension/release factor that can make a good song great.
Compress When Needed. Not Because You Can. Before you slap a compressor on a channel or bus, ask yourself: Why do I need to use a compressor and what am I trying to achieve by using it? If you're trying to tame transients or add a bit of "glue" to a bus, start with subtle settings first. If you have no clue how to use a compressor, learn how before using one. Did you know some saturation/distortion plugins (ex. SoundToys Decapitator) actually end up mimicking a compressor when used by attenuating transients? True story.
The one most important piece of advice I can over is practice, practice and practice more. Training your ears is half the battle.
And I'm spent.
