As far as I'm aware (and someone will correct me if I'm wrong) but "soft" IVF is where less stimulation is involved so you only produce a couple of follicles.....or it's on a natural cycle but instead of allowing natural ovulation, the egg is collected and fertilised with the sperm and put back, as with IVF.
There is an active running thread for ladies having Natural IVF...
I understood this to be like "Mild" IVF - where you use less drugs for stimulation, and produce maybe 7-8 follicles instead of 15-20. Create clinic offer it, and I've been trying to find out more about it myself. Let me know how you get on?
also, as Minxy says, you can do "natural" IVF which follows your natural cycle completely and just goes with the one egg. Success rates are lower for natural, but are comparable for mild vs "normal" ivf at create... but as I say, I'm still researching this one