I found that once I hit 'Master' status with my restoration spells it was easier to just fork out gold and train it up 5 times with each time that I leveled. I know for certain that I'd done this with two of my character levels (ie - I'd go to the restoration trainer and train my restoration skill 5 times - the max). Anyway, so I hit level 90 in restoration, and I wanted to be sure that I'd trained it up 5 times before I leveled my character again, so I went to the master restoration trainer (I forget her name) in Whiterun. But for some reason she didn't have the chat option visible to let me train up with her. I could only assume that it had something to do with hitting 90 (like some sort of cap on paying gold to train up). Is this the case? Once you hit 90 can you no longer train up a skill?
That is correct, you can only train spells and skills up to 90. Beyond that you'll have to do it the hard way and use the skills.
However, there is an one time use item called the Oghma Infinium that can potentially provide a +5 boost to restoration if you find training it up to 100 takes too long.
And there is also a related glitch if you want to abuse it to get it to 100.