Important things to understand about making the currency stronger is demand and supply. There are some good ways to increase the demand. In order for currencies to be strong it must have a value, where other countries would want to buy your currency. If international businesses demand for specific currency to conduct their business then they will demand for that currency. Another way is to have a currency agreement among countries, it sounds like it is similar to currency swap but it is not, it is a lot more like currency futures contract.
Lets say there are 3 countries that are struggling with their currency getting weaker day by day against country 4 with currency Z. We will name A, B and C who are struggling and they want to find a very fast solution to increase their currency. A can sell their currency to B in exchange for their currency, when A demand for currency B the demand for currency B increases and at the same time demand for currency A also increases because A is giving away their currency. With the similar method A can sell their currency to C and B can also sell to C, that way their all increase the demand for each others currency. When these countries trade each other currency they have to be some kind of an agreement so that they can avoid currency speculation. For example when A sells their currency to B, then B cannot give back the currency B back to the country B right away, it should be minimum of after about 3 or even 5 years. By the time the maturity of the agreement gets to the time that they can give back the currency then country B might not even have any currency A left, if they do not have currency A left that is a good sign, because it shows that country A utilized the currency A and made some profit out of it. The contract should also have the fixed rate in the future, so the price of each other currency should be fix if one country wants to return the money back. This method will help most of the countries to increase trade among each other and have good relationship in terms of business and tourism. They would not have to depend on the third currency.
Foreign Direct Investment
Countries should attract foreign direct investment to increase the value of their currency. The more the foreign companies do their business in your country then they will demand for your currency which will increase the demand. Then the country can use the money to run their economy but best advise would be to invest in another country to diversify the income.
Investing in other countries
A country should support the local companies that has the best potentials to run successful business in other countries. These companies should invest in other countries and conduct their businesses there. The money that these companies earns will be in foreign currency and when they send some portion of their profit back to the parent country, then they will be an increase in demand for the currency of parent country.