I approach obfuscation more as an excersise in terseness than perhaps unreadability (although that is a byproduct).
I do them as a mental excercise to keep my brain actually having to think about things i don't normally have to - i agree with djantzen about it being partly "mental masturbation", but at the same time if people want to do that then who's to stop them? And by writing hard code in a terse manner i now understand functions i wouldn't necessarily have used or certainly understand as well (vec, pack/unpack, splice, map, grep etc etc)
Also it's nice to make something that people are surprised and pleased that it can actually be done in that space.

I started off doing them entirely for email signatures, so i almost always use a 288 char limit - 4 lines of 72 chars each.
I use Vim and a script which formats the code into 4*72 lines, removes whitespace and counts chars for me
(you can see it in my scratchpad - usage sizer <name of script to check> <char counting 0|1> <chomp whitespace> <width>).

The process usually starts with an idea - something i've seen that i think i can do in that number of chars (or more likely can't do - but want to try). I'm starting to do bigger ones - like Balls! as the problems i approach get harder to fit in.
For example: The first one i posted here was this - a pair of sigs to generate and then solve a maze. This became an obsession and i kept working at it till i got it into 4 lines.
After the idea i tend to write it in semi-long hand, then start shortening it - spotting shorter ways to do things etc etc.

er, that's it really!

apologies for the rambling post!
al