So show the players the effect of the plot even if they or the plot is not ready yet. That Machiavellian machination plot of country C trying to start a war between Country A and Country B scheduled for levels 11-15: sure it will be interesting when it happens, but the PC's will have more involvement if:
- They find Inflammatory posters and announcements about Country A in country B.
- The hear a sultry bard telling story's in Country B about how decadent/Militaristic/Corrupt/Foolish the people of Country A are.
- They know that the local smith is over worked and back-ordered on military equipment.
- A merchant from country A offers to buy "used" equipment for 2-3x the normal asking price.
- If that sultry Bard in the tavern keeps chatting the warrior up about keep defenses and outpost locations.
- They encounter extra security/inspection/suspicion/restriction as they enter a community.
- They see fields lying fallow or the crops unharvested.
If the PC's cannot "see" the plot's through its effects upon the world then don't bother with it.