I have a King curved soprano which only goes up to D#/Eb.  I can overblow a high G (thumb and 123) to get a D above the staff.  From there, I work my way up the normal cromatic scale progression for altissimo.  I use G# to get D#, A to get E, Bb to get F, and so on.

Since I am a tenor player primarily, the reed/mouthpiece selection is very critical for me to hit altissimo.  I use a 3 or 3 1/2 Rico plastic coat and a Claude Lakey 6 hard rubber mp when i am going to play altissimo.  When I am going to play in the lower registers, I will use a 2 1/2 reed to be able to easily hit low C.  The 3 1/2 is too stiff to hit that note without blasting for me.

I cannot play altissimo on the very vintage Conn mouthpiece that came with my soprano.  I'm sure there are mouthpieces better than my Lakey for hitting altissimo.  I have talked with Mr. Steve Goodson, saxquest.com, and Mr. Dave Jary, jarycustom.com, about mp and altissimo.  Both are super nice and very wise.  I have mps from both and am pleased with both mps.  There are phone numbers on their sites.

Also, I use a different set of altissimo fingerings for soprano than tenor.  Don't be afraid to experiment with different fingerings.  For me, not every fingering works the same on each horn. 

http://www.wfg.woodwind.org/sax/sax_alt_4.html is a very detailed site, in terms of number of fingerings shown, for F# to A.  There are links to other pages that list fingerings for different ranges at the bottom of the page.  I cannot get all of the fingerings to work on each horn, just as I'm sure that not everyone can get the fingerings I use to work on their horn.

I hope this helps some.

Best of luck,
Robert Story