Ok, I'm hoping to possibly GM a game of modern-day Call of Cthulhu for a few friends of mine, but I'm kind of stuck.

Simply put, I just can't design the kind of plots that H.P. Lovecraft and other Cthulhu Mythos writers wove. I've got the book "Tales of the Cthulhu Mythos" (and I'm at the point where I just finished "Dweller in the Darkness," awesome story btw).

I was thinking of having a sort of "X-Files"-ish adventure to start out with involving the Mi-Go, to introduce their characters to the Things Which Cannot Be Named and all that, but I don't know where to go from there. The kind of horror that H.P. Lovecraft and others evoked in their stories isn't something I can easily duplicate.

Does anyone have any campaign or session ideas for this game?

Your best bet might be to run a 'closed' rather than an 'open' campaign. Come up with the general idea of the story arc and then stick with it. When the arc if done, the game is done. This helps prevent the 'new deep one every week' that 'open' CoC campaigns tend to fall into. This way you can keep the slimy things in the background for a little while.

I once ran a great modern game game that moved from Egypt to London to my own home town of Weston-super-Mare!

It dealt with the Hermetic Order of the Golden Dawn (a CoC supplment for them was produced, though not by chaosium) who had a Temple, that of Osiris, located in W-s-M.

It was especially good because all the players knew the locale.

If you want more details I can try and post what I remember.

I think you should probably start with a few, single run missions to begin with. Most of Lovecraft's stories don't feed into each other, it's just the same monsters, so tie into that for a while. Make up a situation, either in a place the characters can imagine well or somewhere they've actually been and is somehow creepy. Some monsters are doing something that involve people disappearing. Then your 'normal' people go and try to solve the mystery.

One thing that made Lovecraft so great was he made up very original creatures. If you can, make up your own monsters with their own plans. All they need is a reason not to kill people outright. The reason the stories are scary isn't so much because of the over arching plot (people are disappearing or aliens are going to take over the world), as the small revelations as to the very nature of what the character is up against. Drop clues that the things your characters are investigating don't belong here, and the only reason they're not dead is because the monsters are busy with something else right now.

Mood setting is really important for these types of games too, more so than for any other style of game, IMO. Candles and no electric lights are very cool (actually, when we did diceless CoC, we each had a large candle. When a character died, the candle was extenguished until we were all in the dark.) It would be really cool to play this outside near a farm field or something, but its getting cold out and they might not appreciate that. I would consider doing diceless (nothing spells run like the GM saying it's not even worth the trouble of rolling the dice...) Our GM always had us succeed at reasonable tests.

If you want a specific plot... One that our GM did was a Jack the Ripper style game. The party, made up mostly of prostitutes, street cops and a doctor, were tracking down the killer. They found the victims always had a metal capsule with holes, and inside was a sponge soaked in perfume. Whenever a character died, she would encounter a young girl just beforehand, and coincidentally the doctor would be on his own. So the characters could avoid death by not finding that girl. In the end, we found the metal things gave off a specific scent so the victim could be tracked. The doctor had been hypnotized to react to the scent and the girl together to kill the victim. Lotsa fun, and very creepy. THe only survivor was a cop who stayed decidedly far away from the rest of the party.