Well - your mixer may very well have something to do with it... but before you even get to the mixer, you have to be sure that whatever you're recording is already sounding good going to tape (or hard disk, or whatever you're recording to).
So, a good mic, and a good preamp is the place to start. Once it sounds good on tape (or hard disk or whatever), it should sound good as you mix it - if it doesn't then your mixer may be the problem, or it could be that you didn't really record your tracks well to begin with!
Bottom line is, every part of your signal chain needs to be good (or at least decent) to get acceptable results, if any one area is lacking, then it's difficult to narrow down the problem, since you can't be sure if your tracking well, but hearing wrong, or tracking badly to begin with.
So.... what is your signal chain as you record?? Also a gear list might help!!
Let me know!
Bruce