How should I adapt a recipe if using very new dried yeast versus slightly older dried yeast? I make bread and pizza bases using "fast action" dried yeast (like this: http://www.sainsburys.co.uk/groceries/index.jsp?bmUID=1287396076254 ).

My bread recipe calls for one sachet, my pizza dough recipe for two sachets. I notice that when I have just purchased a packet that still has a few months to go on its "best before" date, I get really lovely fluffy bread and pizza. As the yeast approaches its "best before" date, it gets less and less effective, but simply adding more of it doesn't give the same results - should I change the recipe in some other way? For example, add more sugar, or less salt?

I saw this similar question: Does active dry yeast really expire? but it doesn't quite answer my query.
First off, how do you store your yeast?  Storing yeast in the fridge helps it to last longer; I've had some yeast in my fridge for a year that is still going strong.

As your yeast begins to become ineffective, I would personally get new yeast.  Once yeast looses its power, you're simply not going to get the same effect out of it.