central banks have their own foreign exchange reserves which they can use to buy their own currency.. then there's this other way of reducing the "monetary-base" - which is the total liquidity or the total supply of money in the system - by selling treasury bonds or any other government securities to large financial institutions (like banks) and hence effectively reducing the supply of money in the market (each bank has an exchange settlement account that it maintains with the central bank.. the central bank debits that account by the value of T-bonds that it sells to a particular bank, and hence that amount is no longer available to those banks then, which effectively means that monetary base is reduced..! but reduced monetary base also means higher interest rates - which also means lower growth, thats where the "monetary policy" comes into play (H)) ...
but bear in mind that these are just corrective measures only meant to "smooth out" excessive fluctuations in the value of any given currency... under "floating" or "flexible" exchange rate system (as opposed to fixed exchange rate system), the central banks' intervention is kept at minimum... its mainly the market forces that determine the value of any given currency..
As I said earlier, there are a number of other reasons which could also influence the value of any given country's currency..