This is, possibly, one of my favorite tricks ever. I'm not really sure why, but I love having it in my back pocket.
So, what exactly is "beurre manié"? Well, it's French; French for "kneaded butter". Traditionally, it's a thickener, much like a roux, but it's added near the end of the cooking process. I make it up and keep it on hand to make my roux (rouxs? Darn the French and their complicated plurals! -- complicated language in general, really...) in a jiffy with no measuring.
Want to make some?
Take a 1/2 c. softened butter (one stick) and mix it briefly but thoroughly l with 1/2 c. flour to make a loose dough or paste. Scrape this mixture onto some plastic wrap, roll into a uniform log, and place in the fridge to firm up. Once firm, cut into 8 equal pieces (easily achieved by cutting in half, cutting the halves in half, and then cutting those quarters in half). Each slice will give you 1 Tbsp. of flour and 1 Tbsp. of butter with which to start your white sauce or gravy.
I stick mine in a ziplock bag and store it in the freezer, then pull out as many as I'll need for a recipe. So, if my recipe asks for 3 Tbsp. butter and 3 Tbsp. flour to make a roux, I toss three pats of beurre manié in there and continue exactly as I would if I had measured out the ingredients individually -- cook until golden and no longer floury-smelling, add liquid, etc.
And if you find your sauce isn't thickening, as I sometimes do, throw in another pat and watch the magic happen -- no clumps, guaranteed! (Be forewarned, though, that this may result in a slightly floury taste, since the flour wasn't cooked.)
This also has great potential to help out a slow-cooker dish that didn't thicken up as much as desired.