Well, I might be wrong about this, but I've found a few things that seem to help, for me at least. It's a bit of an art, isn't it?
Maybe try ensuring the water is at a really vigorous boil, a proper rolling boil, before the pasta even touches the pot. I've heard that helps set the starch on the outside quickly. And perhaps give it a good stir not just when you add it, but pretty consistently for the first minute or two. That's when it seems most prone to welding together, I think.
I also try to make sure I'm not overcrowding the pot. If there's too much pasta for the amount of water, it might cool the water down too much and not allow the pasta enough space to move freely. And sometimes, I wonder if draining it and then letting it sit too long without sauce is the problem. Maybe toss it with your sauce, or even just a little olive oil, immediately after draining. Just my thoughts, of course.