I'm a fighter with a reach weapon and Combat Reflexes. At 4th level, I took the Improved Unarmed Attack Feat. I kept having opponents step to me and they'd be safe from my AoO's. It was really pissin' me off.
I also got the Path of the Sword book for my birthday. It's one of the Legends and Lairs books from Fantasy Flight. I really like it. There's a feat called Pressing Attack. If the opponent takes a 5' step out of an area that I threaten, I can automatically move with them to keep them in my threatened area.
If I had to do it over again, I think that I'd like to have Expertise and Improved Trip instead. Now that I have these feats, the DM doesn't make the enemy NPC's take 5' steps so much anymore.
The recommendation for Weapon Specialization is a good one. You already have the prerequisite feat, Weapon Focus. Get that feat, put two hands on the reach weapon and it'll make that 10 point damage threshold for Knockdown happen more often.
I like the idea of Cleave too. With a reach weapon, you could have more enemies to attack than if you only had a 5' threatened area. Also, you can continue to build on that feat chain with Great Cleave.