I just did this a couple of weekends ago. I did the fronts by myself, and I had minimal help from my roommate on the rears. It's not too difficult a job to do. I think the hardest part was loosening the lower nut holding the rear shocks on. Breaker bar + pipe + wd40 did the trick. The trickiest part was making sure you align the upper spring seat on the rears to match up with the mounting points. (Once you take your old shocks out, you'll see what I mean. You can also go to
http://www.nissanperformancemag.com and look up their 240sx project. Go the the update with the suspension change, and they have a link to an older article where they replaced the shocks and springs. (February 2002) if I remember correctly. That article is helpful, and it's what I used as a reference while doing my shocks/springs.
Good luck and rent/borrow spring compressors from Kragen, Autozone, etc.
Cheers,
C