Someone (Lord Gwydion, I think) pointed to this post on super-simple combat maneuvers. Not a long post, but I'll summarize: roll attack as normal without modifiers, effect happens on a critical hit, defender gets to choose either damage or the maneuver effect on an ordinary hit. It fits well with my own philosophy; I almost never use modifiers for special maneuvers. I don't use crits, though; instead, I use the damage roll as a situation roll (5+ means the effect happens.) That makes special effects much more likely. I think I'll add the bit about the defender's option to take the special maneuver's effect instead of damage on 4 or less to my own house rules, though.
I wonder if there should also be an option to take a special result on an ordinary attack, though. I have already suggested injuries to specific body parts instead of general damage -- basically, "Limbs Shall Be Splintered!" Why not add knockback or trip results as an option? The limitation, though, is that the player must call the special effect before the damage roll; on a 5+, the player's character takes both the 5+ damage and the knockback or other bad effect.