When to use flash cards: memorizing words/vocab, numbers, equations, names, dates, and verbatim facts or lines.
When to use mnemonic devices: memorizing steps of a process or sets of facts/information/people, especially in an order.
When to use concept maps or drawings: learning (rather than memorizing) relationships, processes, concepts, systems, etc.
When to use tables or charts: learning or memorizing systems (eg. conjugation in a foreign language), sets of sets of information (eg. people and when they lived and what they did), and other large/complex groups of information.
When to use songs: for learning or memorizing anything.

Disclaimer: this is a general guideline. If something else works for you, do it!