I'm making this as we speak for tomorrow, so I thought I'd share.
It's simple, and should be quite customizable to your tastes...
5lbs red potatoes
6-10 hard boiled eggs (or omit if you don't like eggs in potato salad)
8-10 stalks celery
3-5 scallions (white and green portions)
Mayonaise (I use Hellmans)
Salt and pepper to taste
Boil potatoes until fork tender and peel, I find it easier to peel them after boiling. Boil and peel eggs (again, I find the eggs easier to peel after boiling...

) Chop celery and scallions and toss together in large bowl. Once potatoes and eggs are cool to the touch, chop and add to bowl. Mix together with mayonaise and salt/pepper to taste. I generally use about 1/2 to 3/4ths of a 32oz container of mayo, but your mileage may vary. Add mayo until it's to your desired consistency. It's ready to go as is, but I generally refrigerate it at least a few hours to chill it.
I've never really experimented with it much, but I'd imagine it's a good base to add other ingredients to or season differently. I've been tempted to add crumbled bacon and to throw in different seasonings like caraway or cardamom, but I'm afraid it'll ruin it's simplistic charm. My grandma and mom used to make this with regular white or yellow onions instead of scallions, but I don't care for regular onions raw, so I changed it up. Freshly ground black pepper makes a discernable difference if you've got a pepper mill!