I recently posted about danger vs lethality in RPGs. For 13th Age specifically, I don't think that greater lethality is necessary, but increasing the danger (risk associated with combat) would add more tension to the game. I've been running pretty difficult encounters that, while they may be "hard," usually don't result in harsh consequences outside of Recoveries burned. With a risk of lasting wounds I wouldn't feel so obligated to increase encounter difficulty, even for my group's super-defensive party.
I think the first step is looking at the lasting wound rule that's actually in the book. It's an optional rule, and the results seem like they'll work pretty well (the caveat being that I've never actually used this rule). Basically, if you take a wound your max HP is reduced by an amount equal to 2+your level. The problem with this (at least in my party), as well as with the homebrewed rules linked to in my danger vs lethality post, are that they trigger when you're down to 0 HP (or making Death Saves).
My group illustrates why this might not be the best way to trigger a lasting wound - they rarely drop thanks to the prodigious in-combat healing provided by the Cleric (and the incredible defenses of the defender-specced Paladin with a magic item and a racial power that enhance those abilities). But the problem isn't just that it wouldn't come up that often in my own games, but that it inflates the value of a Cleric (or other healer) in a game designed to get away from "needing" a healer. I don't really want to go there (especially given that I fall very firmly in the "you shouldn't need a Cleric" camp). So what other mechanical event could potentially trigger a lasting wound? Critical hits. Best part about critical hits? They're random (so that can happen any time, no matter the fight's difficulty), and there's really nothing a healer can do to prevent them (a PC at full health can be critted just as easily as a PC near 0 HP).
Triggering a lasting wound on a critical hit was actually suggested by one of my players, and I fully agree with this over the 0 HP trigger. It also makes more sense given that the critical hit mechanic models a particularly solid, well-placed blow, whereas HP can represent fatigue, will to fight, morale, etc. as well as physical injuries, so running out isn't necessarily supposed to mean that you've taken a nasty injury. The specific idea that my player suggested was that when you're subject to a critical hit, the attacker rolls again (sort of like the confirmation roll for a critical in 3.x/Pathfinder). If the second attack also hits with a natural even roll, you take a critical injury. Actually he had 3 ideas (one involving saving throws and the other the magnitude of the hit as compared with HP), but this is the one I liked the best, and the one I'll likely use as a starting point in my game.
Cool, we have our trigger, now what does a lasting wound mean? The rule presented in the core book does a great job of modeling the fact that an injured PC won't have the same amount of stamina that a hale PC would. The rules I linked to in my danger vs lethality post (using negative backgrounds as a base) serve a completely different function, though. The negative modifier is applied to any skill check where the specific injury would be detrimental. Jumping over a pit? That'll be tougher with a leg or foot wound. Ideally I think negative backgrounds in combination with the temporary reduction of max HP does the best job of mechanical representing an injury.
With regard to determining the severity of the negative background, the natural result of a failed death save doesn't really apply, but I'm thinking you can use the same principle with the confirmation roll. It would shake down like this - the higher the even roll for the confirmation, the larger the penalty. A natural 12 would be a -1, 14 would be -2, 16 would be -3, 18 would be -6, and 20 would be -5. Regardless of how many cumulative lasting wounds you have, the negative "injury" background penalty can never exceed -6. The reduction of max HP due to cumulative lasting wounds would probably work exactly as it already does in the core book.
In addition, it might be fun to have the dice determine what type of injury is sustained as well. Instead of referring to a critical injury table (like in Edge of the Empire), you could simply roll a d6 after taking a lasting wound. The result of the roll would determine what body part was injured.
1 - left arm
2 - right arm
3 - left leg
4 - right leg
5 - torso
6 - head
For simplicity, cumulative injuries would simply add on to a single negative background.
Finally, there's the issue of healing a lasting wound. The rules in the book of "you're completely rid of it after a full heal up!" seem a bit too easy to me. Obviously the specifics would require a fair amount of playtesting (and would vary based on the style of game a given group wants to run), but as a first start I'm thinking this. Whenever the group takes a full heal up, make a Con check. If you fumble, the negative background gets worse by 1 point. Hit a normal DC for your current environment and it gets better by 1 point. A hard difficulty reduces it by 2 points, and a very hard difficulty snares you a 3 point improvement.
As far as the reduction in HP is concerned, you'll have a single injury's worth of HP reduction (2+level) until the negative background is completely gone. If you have cumulative wounds, every time you succeed at your Con check you reduce the HP loss by 1 wound (though you still suffer from at least 1 total wound until the negative background is completely gone).
Again, I haven't playtested any of this, it's just my initial thoughts. For all I know it could get to the point where PCs almost always suffer from a lasting wound, and that's just not a great situation unless you're going for a particularly gritty style of game. If it comes up too often, it will get stale and won't be as narratively meaningful.
Another alternative that I've thought of is handled more narratively, and is based on the "Messy" tag in Dungeon World. The tag basically says that damage is dealt in a particularly destructive way. For 13th Age I would essentially say that certain monsters inflict lasting wounds when they land a solid enough hit. Lasting wounds would emphasize the risk in fighting the particularly dangerous creatures, but would get out of the way when facing more mundane foes.
Ultimately the key is in fine-tuning the trigger. There's a whole spectrum in which lasting wounds can occur, from being very common in a grittier game to not existing at all in a group where the players just don't want to deal with them. That spectrum might look something like this:
Wounds are common - trigger on a critical hit
Wounds are somewhat common - trigger on a natural even confirmation hit after a critical
Wounds are uncommon - trigger on a natural 16+ confirmation hit after a critical
Wounds are rare - trigger on a natural 20 confirmation hit after a critical
Wounds don't exist - they're not triggered at all.
At least this spectrum offers an easy method of fine-tuning the "injury dial" if you find that lasting wounds are coming up too often or not often enough for your group's tastes. Such a spectrum would me much less intuitive using a "trigger when you drop to 0 HP" method.