[Cryptography] Compression before encryption?

[Stephan Neuhaus]

Dear list,

I have come across the recommendation to "compress before you encrypt", 
on the grounds that this makes plaintext recognition through frequency 
analysis much harder.

However, compression algorithms surely have easily recognisable headers, 
right?  Also, I seem to recall a paper that did interesting things with 
encrypted compressed plaintext, but I can't recall any details.

So, does any one know what paper I might be referring to?  Or is there 
any other hard evidence (not personal opinion, however well-informed, 
please) that compression before encryption does or does not help?

Thanks in advance,

Stephan