I am always interested in cleaning up my mix as far as possible.
Eq-ing each instrument can only take you so far.
Too much of Stereo Imaging can ruin a track.
Few people have suggested compression.
And to be honest I've never really used Compressors.
Could anyone tell me what kind of settings(such as Threshold,Ratio,etc.) I could tweak to get a cleaner sound
I have very basic knowledge on how Compressors work.

Compressor is not a tool I would suggest using to get a 'clean' mix and also, it is one of the most misused mixing tools. Compressor is a tool that enables you to control dynamics of your audio file. Let's say you have a vocal that has the last few words of a phrase louder (example: this phrase is VERY LOUD - you could use a compressor to make it like: this phrase is very loud - you could also do the same by some simple automation). There are of course, many more ways and reasons to use a compressor like emphasizing attacks of transients and so on.

Anyhow, a compressor is a very useful tool for controlling audio dynamics, but the MOST important tool to get a 'clean' mix are your faders - if your levels are not set good, no amount of EQ or compression can help you. After you set your levels,  you can jump to EQuing elements and making them sit together nicely.

If you are only starting to learn compression, before even opening a compressor on a channel, ask yourself: why am I using a compressor here and what am I trying to achieve with compression on this particular sound/audio clip.

You also asked about various settings you could use and my advice is: don't listen to everything someone tells you - for example I can write: use 30ms attack and 200 ms release on a kickdrum - you open a project, try it and it sucks, why? Well it's because all the settings are really program dependent. That settings might work for one kick drum but will sound bad on 10 others. What I'm trying to say, if someone tells you use X settings on X sound - use it only as a guideline or a starting point. Listen to what happens when you change the settings and focus on hearing how certain parameters affect the sound.

If you need information about various compressor parameters or anything else I'll gladly share my opinion / post some links.
Hope I helped.

The attack is how much time the compressor will delay its activation... *snip*
The release is how fast the compressor level returns back to 0db... *snip*

This is simply not true.  Please see the "understanding compression" thread.

I disagree with compression not being a cleaning tool. It depends however we define "cleaning up". With compression you smooth things out and have elements pop out of the mix resulting in a better defined mixdown and less messy. In the end it's all about several smart decisions made to get a great clean mix.

Settings really depends on the sound, you have to experiment with the settings till you get the best one. As mentioned above, compression is one one of the most misused tool. Always make sure the input level matches the output level on the compressor. Once you have your settings, bypass it on and off to see if it makes things better. If things sounds worse, redo or completely remove.

Compressors are a mathematical representation of what membranes to do loud sound... they apply a dampening force at a certain amount (ratio) once a threshold is exceeded.

For cleaning up sound, you want to be looking into multi-band compressors, and they are a great way to highlight the particular characteristics of an instrument or voice. See the Multi-band Compressors thread (http://theproducersforum.com/index.php?topic=454.0)... someone might drop hints of how they use them.