I've had the best success with having a tagline for each character: something they say that is very much in character. By saying the line, I slip easily into the skin of that alter ego.
Have you tried implementing the fate system in a D&D game?
I remember that my character is a sneaky thief, or a noble knight, or a steely-eyed gunfighter, or a drunken swashbuckler, or a starship pilot, and I am not, and react accordingly.As a player what steps if any do you take to remember to roleplay how your character would react as opposed to how you would react?
The character is who he is in play, not what's written on his character sheet, so whatever the player does is the character.As a DM what steps if any do you take to remind players that they're playing as a character and not themselves?
Myother personalitycharacter usually steps in and reminds me...
"A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines. With consistency a great soul has simply nothing to do. He may as well concern himself with his shadow on the wall. Speak what you think now in hard words, and to-morrow speak what to-morrow thinks in hard words again, though it contradict every thing you said to-day." - Ralph Waldo Emerson, "Self-Reliance"As far as remaining 'true' to your character ... that is a grey area as far as I'm concerned. I know as a person my character is not constant. It's variable and I often contradict myself. So i think PCs can and should be equally complex and changeable.
The character is who he is in play, not what's written on his character sheet, so whatever the player does is the character.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.