Strategies and Tips: What to do when stuck on math?

This, probably, is a more appropriate answer to: "Strategies - What to do when stuck on some math problem", but I thought it was still worth mentioning.

I've always thought that section 2.1 of this book is a rather nice and concise list of concrete advice regarding how to solve problems in analysis, particularly useful--I think-- for students who might not have a large experience in tackling proofs. If I'm stuck on a problem, having a read through it, sometimes, gives me an idea of what else I can try.

A probability question that I failed to answer in a job interview

Orientability of $m\times n$ matrices with rank $r$

Are there/Why aren't there any simple groups with orders like this?

Fibonorial of a fractional or complex argument

When is the sheaf corresponding to a vector bundle on a smooth manifold coherent?

Learning path to the proof of the Weil Conjectures and étale topology

What is a good book to study classical projective geometry for the reader familiar with algebraic geometry?

Conjectured primality test

If $AM$, $BN$, $CP$ are parallel chords in the circumcircle of equilateral $\triangle ABC$, then $\triangle MNP$ is also equilateral

Qual question archives?

Examples of mathematicians who lost interest in Math and got interested again?

Second-largest connected component of mathematicians