One of my main opponents is a bretonnian player, we've played dozens of games in 6th edition, and one in 7th edition, with more to come.
In 6th I used many different races, from O&G, Skaven, VC, Ogre Kingdoms, Wood Elves, Dark Elves, High Elves, Lizardmen, Bretonnia, Beastmen, and Dwarfs... (i couldn't make up my mind, did alot of paper-hammer)
My friend has used only Bretonnia for about 3 years now, he loves them to death. He has only beaten me, maybe twice in 3 years...
The secret, already disclosed above, is their charge. Control when, and where, or stop it before it starts and you destroy the one trick pony. The best way to describe 90% of bretonnian players is "Wind up the knight blocks, and watch them barrel forward."
I like bretonnia, and think it is a wonderfully colorful and fun army, it requires skill to beat anyone with half a brain, which I enjoy. Surprisingly the times I've lost is when my friend decides to wait for me to come to him (obviously only works against one of my more melee designed armies) where I have to charge him. Its easy to setup several small 9 man knight units in such a way that charging one results in a flank charge by another. Setting this up puts the other player (me) in a hard place, because if I charge i'll get flank charged which will destroy me, if I don't he can take his time to set up a flank charge anyway, or just charge with two units which is powerful as well. Beware the smart bretonnian general...
As a lizardmen specific way to beat Bretonnians...Skinks, ranked or skirmishers, doesn't matter. Just control his charges if he's being offensive. If he is playing defensive, since we have little range he can do this fairly easily, you can either try to use magic if you have a slann, or your going to have to use a little something extra. Most of the time a direct saurus assualt may end up winning, saurus are kicka$$. To be safe, flank a unit with terradons right away, or try and threaten his flanks with a salamander or something, seperate his knight units. If they work in pairs or worse, three's, they can kill just about anything.
Good luck...