If you have grunts in your map, put most of them on patrol with a few path_corner entities. This gives the impression that they are holding the area but don't know what direction you're coming from.
When running Half-Life with the -dev parameter, bring up the console and type in these codes (one at a time) for useful development information (or sometimes just silly effects).
CODE
EFFECT
impulse 76
Spawns a random grunt in front of you
impulse 101
Gives your player all of the weapons, some ammo, and the HEV suit
impulse 102
Creates some blood and gibs
impulse 103
Displays the AI schedule of the NPC you are looking at
impulse 104
Lists global entities
impulse 105
Makes the player completely inaudible to NPCs
impulse 106
Gives detailed information about whatever entity you're looking at
impulse 107
Gives information about whatever texture you're looking at
impulse 195
Shows the links between air nodes from your position
impulse 196
Shows the regular nodes and connections if used by a large monster, these appear as a glowing smoke trail. Large creatures are incapable of walking down steps so the nodes at the foot and peak are not connected.
impulse 197
Shows the node connections for ground-based humans/aliens of regular size. Best used for finding possible breaks in paths leading around scenery or more importantly, up steps. This smoke trail is a different colour from the large beast trail.
impulse 199
Shows all connections from your current postion, plus makes the actual nodes visible for a brief moment when it's activated. The nodes will appear as a small patch of red and white dots. The connections will appear as a red dotted line.
impulse 202
Sprays some blood on a wall you're facing, working just like a spraying a decal.
