You've all been there: After executing the perfect launch, orbit, and transfer burn, you get to the Mun only to realize that you forgot to add landing legs to your lander. Damn!
So called 'professional' space agencies (NASA, ESA, JAXA, etc.) never had this problem. Why not? Checklists. You should never leave the launchpad without one.
Enough blathering, go checkout the KSP Build Checklist product page for more info. Happy launching!