blimey... well there's 3 main points:
Frequency
Level
Spaceial placement
and you can use those 3 primary things to create mixes. Forget fx for the moment because if it doesnt sound right without fx it aint going to sound right with fx, worse probably cos many fx simply amplify the problem.
however, there is another phenomenon which is the way 2 or more sound interact to create sympathetic harmonics and other effects, and often the problem with mixing is that 2 things you begin with sound great, but dont seem to work so good when you add in other stuff... somehow you gotta work thru all that
anyways in the most basic scenario, you have a kik & bass.... if each is allowed full freq they will counter each other, muddying frequencies of each other, so in a basic way you can take a kik & bass and for example lo-cu the kik at around 60hz for example, this gives the bass (assuming it's a LO bass sound) the space below the kik from 60hz down to say 40hz where your mix tails off in terms of frequencies your speakers can reproduce (or the limits of the actual plugin or hardware units usuable range)
ok, then you can go down the bass with a cutting filter and sweep the frequencies from say 200hz down to 80hz, and see how that allows the kik a space to push thru
in that way, using the old rule of utilising eq to CUT rather than Boost all the time, you can creaqte frequency spaces wich suits the 2 items.
then you have spacial stuff, panning in the ordinary sense and stereo panning fx where you delay a sound by a few MS and send the delayed signal one side and the non-delayed signal the other side and it creates a wide spread allowing room for your non directional sounds to sit in the middle
I'd say it's good to work on mixes with no fx... try and get them sounding good as basic mixes without fx. and remember the room/speakers must be ok or all sorts of possible audio anomilies can make a mix sound bad and it's just the room/speaker setup causing the messyness & lack of defination
lastly, a mix doesnt have to allow all items to be heard clearly to work properly. Sometimes some items will NOT be heard so clearly, but they are working in the mix to create the whole sound, so dont be obsessed with clarity of all components, that's a big noob mistake.
I had an idea for a script once. It's basically Jaws except when the guys in the boat are going after Jaws, they look around and there's an even bigger Jaws. The guys have to team up with Jaws to get Bigger Jaws.... I call it... Big Jaws!!!