Moogie, if the data is just a couple of numbers and a short string, compressing the data will make it larger.
of course that is true, however the OP has stated large amount of data... so if there is any simple correlation within the data then simple GZip streams would work as a quick and simple starting point
Elaborating on Cas's idea... if you know the probabilities of the data then huffman encoding... or better yet arithmetic encoding might yield better compression.
How much data are we talking about? What is the context of both the data and the situation that the data is being sent in? compression only really works when you know the data.