1. The numbers adjacent to your top left and bottom right should be as close to those numbers as possible.
2. Placing equal numbers on a (bottom-left to top-right) diagonal does not reduce your options in any meaningful way, so it should be aimed for.
3. Be Lucky.
4. Win with pride but lose with dignity
Diagon Alley isn't just for Harry Potter
Create a middle diagonal (bottom-left to top-right) of tens (plus or minus-ish). That will divide the board into places for 1-10 and 10-20. A place for everything and everything in it's place.
Go further, create 3 middle diagonals: 5-10-15 (plus or minus-ish). Such an orderly room. Now put away all your toys.