Phoenix Wright: If something seems totally useless and you never use it, it's probably the key to winning. :L
If a choice seems really ridiculous or dumb, it's probably the right one.
Shin Megami Tensei (series as a whole): One wrong move and you're dead. Actually, you're probably gonna die anyway. Get used to that Game Over screen.
Buffs and debuffs are wonderful things.
Leave Mitsuru at the base of Tartarus (oh the MC's about to die I could heal or...*casts Marin Karin*)
Fate/Stay Night (VN, close enough): See Phoenix Wright, Rule 2.
Mother (series): Get glue and tape. You'll need it to mend your heart after you finish playing. ;_;