Use a vacumn easel.
there must be some on ebay or craigs list
If you are handy , and I mean handy you can make one yourself.
Keep the motar/pump off your enlarging table and away from the enlarger .
These were necessary tools in the Photo Comp Days before Photoshop, I have used wall mural vacumns for giant murals right down to 8x10 inch V - easels.
If you get one then black magnets strips will allow you to make an edge for aligning the paper into place and away you go.

Quote Originally Posted by AlexG View Post
Whenever I try to make a border-less print without an easel, the paper has a tendency to buckle in the middle. Although it's miniscule, the slight variation in the height is enough to throw the focus off.

Any tips on how to get a good flat image without having to get a sans-border easel?