My alchemy skill now says 150/150, and I don't get any more experience for making potions, even yellow and orange ones. None of the trainers are offering anything when I ask them to train me though!
Easy... You go find the trainer that is offers above you. The levels are simple, such as Expert and Artisan... But some areas don't provide you easy access to just level up... You have to track them down... Such as once you reach the top level for Engineering that you can level from Ironforge, you gotta go to somewhere like Gadgetzan to get the next level... So just scope out the next level up...
Alchemy trainers are in Undercity to get past 150. And in advance, to get above 225, you need to speak with an Orc in Stonard (Swamp of Sorrow). Speaking to the who can no longer teach you will give directions to find the ones you need to advance to next tier :)
Also, to the left is a menu, scrolling down shows you a handy link ;)