One thing I use as a guideline is the Rules of 7's
http://www.dpca.org/BreedEd/ruleof7.htm
However, I seek to expand on them and exceed each of those rules. IOW, I don't limit myself to 7 people or 7 challenges or what have you. I try to do many more than 7 of each of them. It pays to be creative with them - I've literally fed puppies on top of my dryer out of a frisbee, for eg. In the bathtub out of a roasting pan ...
The more situations puppies face and the more people they meet and the more sounds they experience, the better.
I play CDs of thunderstorms, jungle sounds, marsh sounds, sound effects, etc. I build them a play land with tunnels, tiny A-frames, buja boards, mirrors, teeters, ramps, etc. They start playing on these things at the age of 4 1/2 weeks.
That's just a few of the things I do.