I do all my own mixing down and mastering the now, will share what I know!
When it comes to mixing down, most mastering services ask for at least 3db of headroom.
I usually have 6 to 8db before I start mastering them.
Depending on the track:
Drums usually are between -11db to -14db.
Sub between -13 to -16db, you want the sub under the drums unless you sidechain the kick for emphasis.
Main synth -14 to -16 db so it comes out more than the atmospheric one which is around -18 to -24db. ( Can have one or more synths so have to figure which should come out in certain parts of the track.)
Fillers around -17 to -19 db.
White noise/sweepers -24 to -30db.
It's quite easy to achieve this in Ableton or Cubase, not too sure on Reason.
Limiting and compression helps but I don't suggest limiting synths. ( Destroys dynamics )
Want everything to come out pure.
Then I boot it through to Wavelab and start mastering which takes ages.. ( Taking into account the Noise War.. )