>>235
I think it depends. Generally I think the best way to learn maths is to do it until you develop a gut feel for it. Linear differential equations are normally not too hard to develop a feel for, because you'll run into many of them in standard practical applications. Sometimes you'll learn standard ideas for bringing equations from one form into another.
I don't think flashcards would be useful for this.
Nonlinear differential equations are much harder. Sometimes they're unsolvable. When they are solvable, there is no general method which will work for other nonlinear equations of a different type (which you can always do with linear equations, by contrast). This means that the field is replete with clever trickery which you either know or you don't. For example, you either know that the viscous Burger's equation can be handled by the Cole-Hopf transformation, or you can go and beat your head against a wall until you figure it out on your own. Once you know that, it might help you with other nonlinear differential equations (e.g. the Ricatti equation) but it will be utterly useless for others (e.g. the inviscid Burger's equation, or Liouville's equation, although a similar substitution works for the latter, or many other equations which will not be solvable by any trick you can think of).