This paper from Jürgen Abel and Bill Teahan presents several preprocessing algorithms for textual data, which work with BWT, PPM and LZ based compression schemes. The algorithms need no external dictionary and are language independent. The average compression gain is in the range of 3 to 5 percent for the text files of the Calgary Corpus and between 2 to 9 percent for the text files of the large Canterbury Corpus.

ftp://137.193.64.130/pub/jpeg/pfp.zip