Clues

Evaluating the clues you found and making sense of them is fun.
Having a hard time finding them is not.

Some people will disagree, but this is what i experienced, most of the time, as a player or GM.
Players tend to get frustrated and bored if they don't know where to look and fail advancing the story.

So make your clue gathering easy and obvious. Be generous when handing them out and don't make it unnecessarily difficult for your players to find them.

It does not mean they should not be working for them. Of course they'll have to talk to some NPC's, search places and come up with ideas. But try rewarding any idea they come up with.

It leads me to the "floating clues" as they are explained in Trail of Cthulhu.
It's a very clever concept that has helped me a lot since i realized it.
Some clues, as many as you can afford, should not be locked down in a specific place, event or NPC. You should keep them in the back of your mind or on your notes, and place them, during the game, wherever your players will be looking.

Example: The adventure states that the widow Smith will tell the players that her deceased husband joined the expedition to Africa in 1923. If the players don't talk to her, they will never find out. If this is an essential clue, players will probably be stuck until they have the idea to talk to her. It also gives them the feeling of being railroaded.

If you think about it, there are many possible sources your players could get this information from. They might read it in the newspaper archives, they might learn it from one of Pr.Smiths students, or one of the his co-workers at the university, or they might find a letter in Pr. Smiths desk, a picture on his wall. It's not important where they will get the information from. So make it easy for your players. Reward their ideas by giving them the clues they need to advance the story wherever they cared to look.
This is easier if you keep your clues floating and not locked down somewhere.

It's a good way to avoid looking railroady. Even if the adventure actually is. The players won't realize it because wherever they are investigating, they will be able to advance the story. You will be able to steer them where you wanted them to go in the first place by still giving them the clues that will take them to the next part.