Songs (including some traditional folk-songs and children's songs) and poetry can be helpful because the rhyme/meter can sometimes give you some clues to help remember the declension endings:

Расцветали яблони и груши
Поплыли туманы над рек__
Выходила на берег Катюша
На высокий берег на крутой.

Here, the word for "river" has to be in a case that rhymes with the end-stressed masculine adjective кру-ТОЙ -- thus, it can only be "над ре-КОЙ" (feminine instrumental, end-stressed), but not "над РЕК-ой" (correct ending, wrong stress) or "над ре-КЕ" (which could be either dative or prepositional, but neither one is allowed with "над") or "над РЕ-ку" (the correct spelling and stress of the accusative singular, and "над" can take the accusative in some contexts -- but it doesn't rhyme!).