TITLE: References for sign conventions in homological algebra
QUESTION [30 upvotes]: There is no shortage of sign conventions in homological algebra.  And once these conventions are set out, there is no shortage of diagrams where an obvious commutative diagram on the underlying abelian groups has to be corrected by an appropriate sign.  Many people take a fairly relaxed attitude towards these.
Unfortunately, the signs don't always "just work themselves out".  Conrad's book "Grothendieck duality and base change" (see his webpage) gives an extensive list of diagrams, and several signs that are likely opaque for most students of the subject initially.  (For example, I find it unlikely that most graduate students would successfully identify the two signs going with canonical isomorphisms $(A[p]) \otimes (B[q]) \cong (A \otimes B)[p+q]$, and the reason they're different.)  Poincare duality and the cap product give another offender.
A number of the signs going with these diagrams have conceptual explanations going with them, but most of the references I know generally present sign conventions (and the signs going into the appropriate diagrams) as dropped from the sky.  I'd like to know if there is a place in where I can direct students who might want to understand this, rather than having to explain myself.
Do there exist references in the mathematical literature giving a conceptual basis from which these signs might be derived?
(By no means am I claiming that I know most, or all, of the relevant literature.)

REPLY [14 votes]: There is a short classical paper that has not been mentioned:
Boardman, J. M.
The principle of signs.
Enseignement Math. (2) 12 1966 191–194. 
For products in homology and cohomology theories, there is a
systematic explanation of the products on the level of the
symmetric monoidal (homotopy) category of spectra that dictates
the signs.  See Section 9 of the following (also on my web page:
http://www.math.uchicago.edu/~may/)
May, J. P.
The additivity of traces in triangulated categories.
Adv. Math. 163 (2001), no. 1, 34–73. 
The literature is strewn with mistakes of signs, which can have
serious calculational effects. I once spent months tracking down
a contradiction of signs involving the Steenrod and Dyer-Lashof
operations. The mistake occurred in the unpublished errata of 
Steenrod and Epstein, which was meant to correct signs in the
original classic text.