Sepia's a very reliable method.
For me I always try to keep the color scheme different. Maybe sepia but for this comic I haven't used sepia. I also include the character that's part of the flashback's current self with their flashback self to show that this isn't the same version of the character and also not happening outside that character's head. Also use a lot of gradients for that fading feeling.
Drawing them is mostly the same as my regular method, just slightly different colors. I want to experiment with different styles in the future tho.