You have to be human to see it, you can't see summon signs when hollow. That's what I use my human effigies for. I let the ring of binding (75% health loss instead of 50% when dying) keep me up while going through the main game, but I use my effigies for summoning/pvp. As for how often I use it, I've used it a couple of times to help people. You can lay down a summon sign outside a boss area (so that they can summon you into their game), help them kill the boss and you get all the boss souls for doing it. It's good soul farming. I've only used a summon to help me in a fight once, and it was the NPC not a real person (you get an NPC to help you later on), it was against the trio sentinels. You'll understand when you get there. Think Ornstein and Smough from the first game but only add an additional boss to fight.
Also, maybe you tried to burn an effigy at the fire instead of use it from your inventory? If you burn it all it does is give you a smaller chance of being invaded by a player. To get your health back/summon/become human, you need to use it from your inventory.