Just prevent crossings other than at checkpoints, require passports so you know who is coming and going and how long they're staying, just the same as any other port of entry. Like international airports for instance.
I'm fine with that. I don't think that's what most folks mean when they say, "seal the border" though. Usually it's more along the lines of, "put up a big wall and shoot on sight."
"America is at that awkward stage: It's too late to work within the system, and too early to shoot the bastards."
-Claire Wolfe
"I got a shotgun, rifle, and a four-wheel drive, and a country boy can survive."
-Hank Williams Jr.