Angelsboi said:
wel i dont have the PCs yet and im just kind planning on things i want to happen. Im still fairly new to Cthulhu mythos myself.
Well if you are setting up your campeign still, then... (witout covering what omnipotent wrote well enough) :
1) Make sure that you set up the adventure so that the PC's want to find out what happens. Like a car wreck on the highway everyone complains about it and knows that it is up there and that is why the traffic isso thick, but when they get there they still sit and stare at is as they pass continuing to make traffic slower hoping for a head rolling on the ground or something.
2) Retain the suspiscion of bisbelife. If you ever give the PC's enough evidence to go to the police and have a clear record, 9/10 times they will (if they do go to the police see omni's way of handeling it in his post). Make it so that the police have no solid evidence of anything that the investigators bring to their attention. If the PC's look crazy it will add to your gaming experience.
3) Let your PC's do stupid things. Most people who are new to CoC find it difficult to make the transition from D&D laws (e.g I walk into the bar and kill the evil wizard. DM: The people look at you with questioning faces untill you tell them it was the evil wizard and they all cheer.) to IRL laws (e.g. I walk into the diner and shoot the leader of the cult and start to search his body for the tome. DM: the people of the Diner run out scraming or hide under their table shaking with fear. A passing cop comes into the diner after hearing the gun shot. PC: I tell him that this is the leader of the evil cult. DM: The cop looks at you with a strange twitch in his eye and then pulls out his gun and tells you to get on the gound with your hands behind your back.) and it will cause some interesting situations

. Also by letting them make mistakes they will learn that CoC is more deadly and have to play smarter. (e.g. I take out my club and start to beat the Dhole. DM: Ummm... well... [looks at the dice] you just died.)
4) Put stuff in the game that they have no controll over. Make them want to help NPC Smith stay alive when they find him in the sewer system. But when an unforutunalte tunnel leads them right into the path of half a dozen Mi-Go kill off the NPC with gory detal and then have the Mi-Go leave for no reason. This also makes a PC go hmmmm. Plan for a few situations such as this and your PC's will most likely live longer and still have a good amount of combat/action.
5) Tomes and Spell books. Dear god I love these. Thow them at your party, but it takes them 3 weeks to decypher. But you know the end of the world is comming in 2 weeks!!! E GAWD!

No seriously it is one of the best props to give your PC's. There are plenty of resources for this online to make them ( I even have one for the "End of Paradise" module in the d20 core book at my website
www.shapermc.com ). The best place for the source of these books is lovecraft himself. Like in the Campeign for Beyond the Mountains of Maddness you find the tome from the previous antartic expedition, which is really just At the Mountains of Maddness by HP Lovecraft.
I hope this all helps, and also I would like to recomend that you run "The end of Paradise" as an opener module. It works really well and it is easy to orginise for decent one-shot game. About 12 hours of planning and reading and creating for about a 6-7 hour game. That is pretty good if you ask me.
Good luck again.