If you do the valiant dailies for each faction you get a ton of rep. Some of us were gimped in the rep department due to leveling well before BC. I started out way behind on rep, but I have quickly caught up. Here is what I did.
1) Do the Valiant Dailies at the Same time as you Champion Dailies: They are pretty much the same quests and you get credit for both at the same time. Very easy. The only ones that arent the same are the jousting ones in the Argent Camp and the one to fly off and find a damn sword.
2) After your main faction, pick the one with the lowest rep next: If you are lowest on Exodar rep, pick them. If it is Darnassus or Gnomeregan pick them. You will get 250 rep for them each time you turn in a valiant quest. You will also receive rep towards each of the other factions when you do this.
3) Choose writs instead of gold: You can turn in these writs with any faction you are a champion with. Each writ gets you 250 rep towards that faction and another 75 towards each of the other factions.
4) Once you can turn in the Valiant Quest, DON'T: When you get a enought valiant marks to complete the quest, hold onto it. You can continue to do dailies with that faction to maximize your rep. Keep doing this until you are exalted, then you can move onto the next one. Depending on how close to exalted you were, you may have enough valiant marks to turn complete the next faction champion quest.
Follow these simple steps and you will be an Ambassador in no time flat.