If your downloading movies from torrents, the quality of encode itslef is going to be the largest factor.
The problem is, when you convert a file (AVI,MKV,MP4 whatever)to a DVD format, you must recode the it so it has a proper structure.
Theres several apps that can do this, (ConvertXDVD being the most simple but also not the greatest quality)
When you go through the re-code process (regardless how good the application is) there is always a quality loss. If the original source is borderline in the first place, theres simply no way of getting around artifacting.
Many also crop the black borders which is, IMHO, the worst possible thing you can do as you gain absolutley nothing other than messing up native resolutions and causing more work for future recodes.
Burning the disc slower will not help sadly.
Also, as suggestted, if its a low resolution: recoding and resizing at higher bit rates may help slightly. Though not much if the original source is of poor quality.