Eric Shoemaker over at Pale Rider has some thoughts:
Chess players have an advantage in a way others do not, and by that I mean that we can often tell when our game is affected by our age. How? The National Rating itself is an indicator. Older players often reach their floor several times and although they sometimes creep above it, they usually return to that floor in a short time.
I myself have not reached this stage as of yet, but nevertheless, I believe I have the solution for it.
I see many of these "older players" or even players showing the same characteristics play openings and defenses that are usually cautious and sometimes dull in order not to be defeated at the outset. Their usual excuse is "I can't keep up with the theory." Yet the so-called quiet moves they play often allow for even more possibilities and greater variations to consider! This is exactly what is bringing them down, for they lack the mental abilities to consider the larger number of variations.
What they should be doing is returning to openings and defenses that have a large body of theory that is already well-known and "unlikely" to change, because it is either unpopular at the present time, or the theory is already well established without much room for surprises in theory.
Read the rest if you're interested in some specific recommendations.