and does it involve citrus? 
-Stephanie Adams

Wow, if I had a dime for every time I was asked this very question, Stephanie, I'd have a small stack of dimes with a very interesting story behind them.

Before getting into how to stop the universe's expansion, you first have to consider if it is expanding at all.  Popular belief is that it is indeed expanding, and  at an astronomically rapid pace.  However, certain theorists believe that the expansion is nowhere near as strong as the inherent gravity of the universe, and that it will ultimately collapse back upon itself, possibly to start the entire  process over again.  For argument's sake, let's assume that's a load of horse shackles.

How can you, a common layperson, stop the universe's expansion?  And how on Earth did you know that it involved citrus?  The plan, naturally, involves the planting of citrus trees (preferably grapefruit).  I'm talking roughly fifty billion of them.  Per country.  As a bare minimum.  The purpose of this is obviously to increase the density of Earth, thus increasing the gravity of our planet.  This will cause the orbit of our moon to go askew, and likely cause a brilliant yet catastrophic collision between our fragile planet and its only moon.  This collision would likely throw off the delicate balance of our solar system, and within a matter of a few years (perhaps as few as several dozen million), our solar system will have, in essence, imploded.  The geological significance of this implosion, as well as the astronomical significance, practical implausibility, and ethic questionability of it, would be seen immediately within our solar system.

Of course, this will have no effect whatsoever on the universe as a whole because, as Douglas Adams so wisely pointed out, the universe is very big.  In order to have an effect on the entire universe, you will need to have finished some precautions.  Such as: before obliterating our own planet with the citrusy death, you must first have gone to every major solar system in the universe and start planting some grapefruits PRONTO.  You probably only need to do this to a few billion solar systems if you're picking the largest, and that should be sufficient to create enough devestating supernovae to at least slow the expansion of the universe.

NOTE: It is very likely that, before planting anything, you may need to dedicate a small period of time (likely several thousand years) creating a hybrid citrus tree that consists of equal parts citrus and molten iron core, as that would probably be the only thing dense enough to actually affect something as static as gravity.

Let me know how it goes, and keep the questions coming!  Fats is here to, with proper manipulation of the word, help.