Freebase Dali

I don't use Ableton, but there are general rules you should follow regardless of what program you're using... that said, are you mastering your own stuff? If so, you're better off just getting things as good as they can be, in the mix itself. There's no sense in fixing something in the mastering stage when it can be addressed in the mixing stage. Besides, (pre)mastering is usually done by someone who was not involved in the mix. It's about getting an outside (and extremely informed) perspective on the strengths/weaknesses of the recording/mix and fixing them and/or adding that final touch to the mix.

Another question to ask yourself:
Was the song recorded in a studio? If not, then it's never going to be studio quality. The gear in recording studios, down to the cables, are generally top notch... not to mention the superior acoustics provided by professionally designed recording spaces. They also use experienced, professional audio engineers, so that's going to make a big difference. So what am I trying to say? Well.. Be realistic in your expectations.
Simply adding processing to a song is the furthest thing from what goes into sound quality than you can get.

Of course, this is not going to stop you from wanting to master the tracks anyway, so your best bet is to do a lot of research and learn "why" processing is used in mastering and how its used. You'll still only be one step forward (if you don't have the proper mastering environment and equipment) but you'll at least get better results than you're getting now.

Check out "Mastering Audio" by Bob Katz. It's a must read for any audio engineer. There's an e-book floating around the web for free. Just gotta hunt it down.