The only way to strengthen a national currency over the long run is to have exports exceed imports. For countries that use another country's currency as their own, that country's imports/exports would be counted as the "host" country's imports/exports

For example:

Country "A" has a very strong economy and it's currency is used widely worldwide.

Country "B" is relatively small and uses Country "A" currency as their national currency (as in they deal solely in that currency, and acquired the currency from selling goods/services to elsewhere.

All else being equal if Country "B" has an increase in exports in relation to it's exports and all else is equal then Country "A" currency will get stronger