You have to stalk multiple WalMarts. There's a summer and a post Christmas clearance. They have multiple tiers of discounting, and then a final one. The final one is 75%-ish of WalMart daily price and 80% of Lego MSRP. At that point there will only be "unpopular" sets left. This year unpopular was Lego Movie 2 and Avengers. Last year unpopular was Star Wars Solo and Ninjago. Plus random unpopulars like a few Friends sets. Unpopular is often correlated to being overpriced per brick, and some of those are actually nice sets.
You can only get final clearance if you are on top of it enough to know their patterns to be able to walk in an hour after they change the prices and get your 75%ers. It's not for everyone. This year I got a lot of sets I liked for 50% off because initial clearance after Christmas was 50%. Usually initial clearance is 20%, then goes to 30, then 40, then 50 on a weekly basis. This year it sat at 50% for 5 weeks, then shifted to 80%, then was gone within a couple hours. (At each store there was me and a couple other groups also on top of this game.) If you really like a set, grab one at 50% and don't worry about 75% since only a small number of sets make it to 75%. The selection will be different from store to store, as will the exact days they change the prices.
In addition to the stalking game, just checking the clearance aisle regularly you sometimes get lucky and are the first to see a set that was "lost" like stuck behind a shelving unit or something and is put out at 75%, or sometimes 90% if it's been like a year since it was discontinued and it managed to be lost that long, and also usually will be dinged up and maybe have a tear in the box. I manage to grab a lost set about once a year. Last year it was the Lego Boost Robotics set I got for $20, down from $160. This year it was the Rexcelsior I got for $40, down from $160.
Target never goes below 50%, but they do go to 50% at the end of clearance and often on nice sets like Architecture. The sets will also sit on the shelf longer so you don't have to rush around so much. Target also sells giftcards during a 10% off sale around Black Friday so you can buy a $100 gift card for $90 and then use it to buy Legos on clearance and get 60% effectively off at the end. They also do a thing where you get a $10 gift card for a $50 Lego purchase during holidays, or something like that, I forget the numbers. Those are all 1 day specials but buying a gift card you get to use it later, like after the price goes down more. Don't quote me on the Target, I only did Target one year as it's really far from me.
Also this last black friday weekend lego.com had tons of sets at 20-30% off including top end ones like Ninjago City and Docks, and both amazon and walmart.com had a large number of sets discounted in the 30-40% range. Probably why everything went direct to 50% after Christmas.