Highest compression for files(for web transfer)?

I have seen some highly compressed files around, such as 700MB of data compressed to around 30-50MB.

But how do you get such compressed files? I have tried using software like WinRAR and 7Zip but have never achieved such high compression.

What are the techniques/software that allow you to compress files so well?

(P.S. I'm using Windows XP)

If time taken to compress the data is not an issue, then you can optimize compressed size by using several different tools together.

Compress the data several times using different tools like 7zip, winrar (for zip) and bjwflate.

(Note that this does not mean compress the zip file over and over, but rather create a number of alternative zip files using different tools)

Next, run deflopt on each archive to reduce each archive a little more.

Finally, run zipmix on the collection of archives. Since different zip tools are better on different files, zipmix picks the best compressed version of each file from each of the archives and produces an output which is smaller than any that any of the zip tools could have produced individually.

You should note however that this is not guaranteed to work any kind of magic on your files. Certain types of data simply do not compress very well, like JPEGs and MP3s. These files are already compressed internally.

This depends entirely on the data being compressed.

Text compresses very well, binary formats not so well and compressed data (mp3, jpg, mpeg) not at all.

Here is a good Compression Comparison Table from wikipedia.