“With a crowbar.”
(Yes, there’s a joke in there somewhere.)
Actually, I think that building a lasagna or making stuffed shells or even manicotti is always a good option, provided you’ve got the right kind of cheese. Do you have good cheese where you live? 
Add them slowly to the water while stirring; don’t dump them in the water all at once.
Haveyou tried different brands? Some have slightly different shapes, and that may make all the difference.
Yes, as stated in my first post, I’ve tried a variety of quality brands.
As pulykamell noted earlier, perhaps I’m not using enough water:pasta.
I also had trouble with nested shells, and accidentally found a solution: instead of all shells, I use roughly 2/3 shells and 1/3 ditalini.
The ditalini seems to out-compete the shells in nesting ability, but are enough smaller that they still let the water get inside the shell so it cooks properly.
Of course you then have a mixture of pasta in your sauce, but that bothers me less than having those paired-up and thus less than well cooked clumps of shells. YMMV
Darn that coding. Water : pasta.
Shells suck anyway. Stupid little cups holding a random amount of water, which ruins the perfect sauce viscosity that has just been achieved separately.
Coupla tricks:
Only stir once or twice, yes, and then leave the pot alone
add salt, but no oil
when you take them out, rinse them with cold water. this makes them not stick. when you are ready to serve, you can either let the sauce heat them up or boil a kettle of water and pour it over to heat them again.
i never get nested pasta shells; not even 1 or 2.
Just don’t use Orecchiette. Those things nest infinitely.
QtM, you’re not stirring properly. You’re getting a laminar flux, a current going round and round but which does not touch the edges of the pot’s bottom.
Change hands, change radius, make sure that spoon touches every damn single square sixteenth of an inch in the pot’s bottom. You don’t have to stir continuously, but you have to stir right.
If you’re not needing to add extra water, your water volumes are fine.
El Hubbo used fusilli/rotini last time with his spaghetti sauce, and it was awesome. They just sop up the sauce, and they’re sturdy enough to stand up to the bolognese. Num.
I dunno that I’d rinse the noodles; that’ll wash off the starch. It’s probably not such an issue with a marinara or meat sauce, but the noodle’s inherent starch helps more delicate sauces cling to the pasta.
The pasta should be being drained from the pot just before it is going to be served. Get the sauce on the pasta straight away. There’s no need to rinse pasta after cooking, unless you are intending to use it for a cold pasta salad.