You can use just washing liquid and water but if you want stronger bubbles glycerine is fantastic (you can buy it at the chemist) or you can also disolve sugar in boiling water and add the dishwashing liquid. Leaving it overnight seems to help make better bubbles too.
At the daycare where I work we all argue about the right amount of each thing to add, its a bit of trial and era to be honest! I normally do about 1/2 cup liquid 1.5-2 cups water and 1 tablespoon of glycerine or 2 of sugar.
have fun 