I am a great fan of the boardgame Pandemic. It's a tense, cooperative game that really brought things up to the next level with the recent expansion. There are a lot of mechanically neat things about it - for example it makes cooperation incredibly valuable (necessary, even) but also makes it very difficult, which really helps the tension. One mechanic in particular really stands out as something that might be reusable in other games.
The Pandemic board is a map of the world with a number of cities marked. As cities get infected, tokens are put on them to represent the diseases being fought. There is an "Infection Deck" which has one card for each city. This deck is drawn from to determine where the initial infections occur and is drawn from each turn to determine where subsequent infections spring up.
All well and good so far, but where this gets interesting is when an "epidemic" card is pulled - a new card is pulled from the bottom of the deck, that city gets infected, the card is discarded, then the entire discard pile is shuffled and put on top of the draw pile.
The impact of this on the game seems subtle at first, but it's the engine that makes the game work. It means that your problem areas are going to keep being problem areas, which keeps the tension ratcheted up. That's good for Pandemic, but possibly even better for other games. This is a great mechanic for systemizing reincorporation.
For those unfamiliar, reincorporation is a technique in fiction used when something brought up early in the fiction comes back in later on. The classic example of this is that a gun introduced in the first act of a story which is guaranteed to be fired by the end of the story. This can take any number of forms from the blatant to the very subtle and is a useful technique for gaming for many of the same reasons it works in fiction. Notably, it means there are fewer things to keep track of (so you don't introduce another gun when the time comes to shoot someone) and by giving those earlier elements meaning later on, everythign ends up feeling more cohesive and planned, even if it's not. This means that, as a technique, it makes things easier for the GM and her players and it makes the GM look smart - what's not to love?
These ideas come together once you start keeping track of the elements that have come up int oyr game. The easiest approach is to use one of the many inspirational decks out there like Story Cards or the Harrow Deck. Draw from it as you normally would for inspiration until you reach a turning point in your game - a big showdown or the like - and shuffle the cards you've already drawn and start again. When you draw a card you've already drawn, you don't need to literally use exactly the same elements you used the first time, but the simple act of hitting the same theme again can make everything hang together in a way that will feel like you planned it all along.
Even if you're not using cards, the idea still holds up well so long as you keep track of things. If you use a 5x5 grid or a similar system, don't cross things off as you use them, just put a checkmark by them. When the time comes, you know you can come back to those. The same thinking applies if you use element lists, or anything else you can keep track of. Whatever mechanic you use, that reminder to reincorporate previous elements can combine powerfully with inspirational randomness to make even the most impromptu game really hang together.
Bottom line: Track the element you introduce, however you introduce them, and find some way to make it likely that you'll go back to the things you have already introduced rather than constantly generate new elements. This will allow you to reincorporate themes and elements and give your game a more cohesive feel.
1- This works equally well if you are drawing cards over the course of the game, or if you are drawing them over the course of a campaign. The only trick to it is that if you are drawing for planning rather than play, you need to store the cards carefully between sessions so you don't lose the order.