In Hitman:Absolution, I am currently playing the mission Part 1: Terminus. I am playing in Normal mode. AFAIK Normal mode does not regenerate Instinct. I have to rely on other sources for that. In my campaign I am real low on Instinct. How can I boost it, to get past guards and to Room 899?
A usual and practical way to restore Instinct is by finding a nearby NPC to kill/knockout silently and then hiding their bodies in a container. Check if there's a container nearby where you can safely hide a body before going in for the kill/knockout. Make sure the kill/knockout is silent.
You can also gain Instinct by "hiding". This can be done by pressing the "hide" button when a prompt comes up near an object where you can hide.
(Thanks to Ben for mentioning.)
If you are in a position where you can collect evidence or achieve a mission objective, then you might want to go for those instead, as doing those will also restore Instinct.
To prevent Instinct from going down, avoid being noticed by NPCs wearing the same clothes as your disguise. Sneak/crouch whenever possible when around these NPCs; this will lessen the need for instinct mode.