First, Polearm momentum works as follows: If you slide the target 2 or more squares it falls prone at the end of the slide.
That's any weapon attack (keyword is at the top of the power description) which should be practically every single fighter attack.
There's only one fighter At-Will that works with Polearm momentum in the heroic tier: Footwork Lure.
Now Footwork Lure says 'slide the target one square into the square you just left,' (thanks to errata) which is a tad restrictive, but the plus side is that you can work with this. As long as the monster ends up in the square you just left, it can travel through any number of squares to get there. And since it's a slide, you can do L shaped travel all you want. Also, you have reach, so you can slide it two squares if you're two squares away.
Now what's the easiest way to get a two square slide? Rushing Cleats. Increases the push or slide of any melee attack by 1.
Rushing Cleats | D&D 4E | Obsidian Portal
P.S. Polearm Momentum doesn't work on Pushes. Only slides.
P.P.S. In the Paragon Tier you can use a Glave (Heavy Blade+Polearm weapon) with Heavy Blade Opportunity (allows you to use at-will attacks on opportunity attacks) to prone at will. Combined with a Paragon Path that gives threatening reach, you can now prone people At-Will any time they provoke an opportunity attack and you hit. With Polearm Gamble, they provoke an opportunity attack any time they enter a square adjacent you. This sets up the 'touch me not' of a Polearm Fighter.