any quick/easy adjustments?

once your car starts getting old, what are some low-price or easy things that you might do to help restore some of its youth's performance qualities? (if that makes any sence.. sorry, craving sleep at the moment)
for example, will changing the sparkplugs make much of a difference to the hp?
also i heard, some of those "special" motor oils that are suppose to help increase engine performance is just a scam..