Quick and Dirty Rules for Trill past lives:
For every lifetime the trill lives they must spend 1 development point, and they gain a free skill at 1(2) this skill must reflect their what their previous host did in their last lifetime. (Note limit of ten past lives per Trill... not recomended for munchkins)
The modifiers to difficulty are not cumulitive, so if a character with 2 resistance takes 4 damage one turn and in another takes six. He would be at minus -4 (4+6=10).