If you're looking for a single, extremely practical thing, I read the book "Design Patterns" by Gamma, Helm, Johnson, and Vlissides. The patterns in the book we

If you're looking for a single, extremely practical thing, I read the book "Design Patterns" by Gamma, Helm, Johnson, and Vlissides. The patterns in the book were somewhat helpful, but starting to think in terms of patterns really did help. Years later, I rarely think in terms of specific statements or calls needed, but in terms of higher level patterns that I can combine. Things like "Hmm.. this situation really calls for a level of reliability and error recovery which normal REST APIs won't provide, so let's combine 'idempotent calls', 'event queues with guaranteed delivery', and 'synchronous wrapper for asynchronous calls' to build a solution."