Generally, and in theory - flatten the contrast, tweak the reds towards magenta and tweak the blues towards cyan.
As an example (in Photoshop) - add a hue/saturation adjustment layer, pull the hue slider left to about -30 and (if desired) pull the saturation slider right to taste (around +20-30?) . Adjust opacity of this layer to taste (around 70%?). Add a curves layer and pull the top right end of the curve down and the bottom left end of the curve up. Adjust opacity of this layer too to taste. Also add a little bit of grain - film in the 70s tended to be grainier than today's film for the same ISO rating. If you're printing, try to print with a white border and on that textured paper that was so popular at the time and is so difficult to scan in today...
I've attached an example of the starting image from Manipulation Challenge #33
with the above techniques applied. It certainly looks more
1970s, but I'm not sure it's entirely convincing.
Also, apparently the free ImageEnhancer
application does something like this if you use its "Old Photo" setting. I've never tried it myself though.