wingnutamj has posted some nice Technic spring suspension. Find it here:
http://forums.nxtasy.org/index.php?showtopic=790&pid=6231&st=0&#entry6231

parts needed:

4 15N technic bars
4 pins (1*2)
6 large pins (1*3)
2 5N studs
4 pin stud things (see attachment)
1 rod extender (put 2 technic rods into it)
3 rod caps (1*1 circular pieces (full size))
1 12N technic rod
1 10N technic rod