Please forgive me if this combo has been discussed before.
Race: Warforged
Class: Fighter
Paragon path: Warforged Juggernaut
Epic destiny: ?
Relevant feats (heroic): Battering Shield (PHB3), Polearm Momentum (MP), Shield Push (PHB), Swift Spear (DR378)
Relevant feats (paragon): Impaling Spear (MP2), Spear Push (PHB)
Relevant feats (epic): ?
Relevant powers: Tide Of Iron
I can use Polearm Momentum with an at-will power, an AOO, a CC attack or a charge.
If you just want to knock prone, I know this is not the most efficient way. I left out some other good feats like Warforged Superiority and Staggering Challenge because they were redundant.