The following method works from level one, as long as you allow Berserker Strength to count as Rage for the purposes of prerequisites.
Human Barbarian (Berserker Strength) 1
1: Any
H: Ettercap Lodge Berserker
F: Aberrant Dragonmark
F: Dragonmarked Rage
Now, how does this work? First off, Berserker Strength triggers whenever you drop below the threshold, but is only removed if you are raised over the threshold by healing, not through means like increasing your Constitution score. Also, Fast Healing triggers at the beginning of your turn. Remember that, it's important.
So, let's say you get dropped to 3 HP on the turn of the guy who goes before you. You immediately get +4 Str, +6 Con, +2 to all saves (+4 vs. Poison), DR 2/-, and a -2 to AC. Your boosted Con increases your HP to 6, which does not end your Berserker Strength because it wasn't healing. With me so far?
Your turn starts - you heal one HP from Dragonmarked Rage. You get healed to 7 HP, which is more than 5 HP, so your Berserker Strength ends, dropping you to 4 HP from the lost Constitution. Hey, what do you know? That triggers Berserker Strength again, giving you that sweet, sweet buff again. You're at 7 HP.
At the beginning of your next turn, you heal 1 HP and have 8 HP, which ends your Berserker Rage, dropping you to 5 HP. The cycle then ends.
At 1st level, the cycle runs from 1HP to 4HP; at higher levels, that shifts up a little (it goes from 3HP to 9HP at 2nd level, for example). If you can avail yourself of a pool of temporary HP or a method of causing minor self-harm, then you can keep the cycle going for quite a bit.
Why is this useful? There are some nice stuff that triggers 1/rage, like Unholy Fury (1/turn Smite might be comparable with Pounce for a low-level build) or Mad-Foam Rager (1/turn "delay an effect for one turn"? Yeah, I'll just use that to tank the infinite damage your d2 Crusader just dealt me.) or Intimidating Rage (it's a bargain bin Frightful Presence!)
