There are many such ways to do this; have a good credit record, not default on loans, have high interest rates, have a strong legal system, have a good business environment, have high quality workers, good infrastructure, political stability, high exports, strong financial system, etc.
Now many of those are long term sort of things, but short term, interest rates, balance of trade changes, political stability, etc.
These are the conditions needed for a strong currency, not "how" to strengthen the currency.
To strengthen a country currency, offer what the world need.
Invest in export and tourism industry. If the country has high debt, reduce it and live within the mean certainly help.
Let the market set the currency rate and the interest rate. If the country want to excel in service type industry, invest in telecommunication infrastructure (high speed internet). If the county want to excel in manufacturing type industry, invest in highway, power grid and low cost power and clean water.
Final point, find the country unique advantage that can be done efficiently and offer the product/service to another country.
If this country already self sufficient, there is little need for international trade and hence no need for strong currency.