Yes, mastering the timing and positioning of your thumbsticks for both out wide and down the line is the key.
I used a created player, lefty, 5'9" (like me, lol) and built him up to about level 18 before I sold off the game. His serve rating was around 80. In the deuce court, I'd serve either a slice ("X") or a flat ("A") serve down the T with the thumbsticks. (If you use just the buttons, I'd recommend going with just the slice serve, as it gives a little more separation and the flat serves just aren't as effective with the lesser pace.) In the ad court, I'd go almost always out wide with a 120+mph flat serve , although the slice also did the job (again, YMMV if using the simpler button scheme). I really never liked using the topspin serve...it's just not very effective or meaningful in this game since aces and placement are so easy.
Now, the key to using the thumbsticks? As soon as you toss the ball with the right thumbstick, wait a split second before pressing the left thumbstick completely in the direction that you want the serve to go and let off of it at about the same time that you hit the ball (at the peak of the toss) with the right thumbstick. Seriously, that's it. It'll take a little time to master the feel of that "split second" and coordinating the use of both thumbsticks, but once you do, you'll be pumping out ace after ace.
Some people have said, "Great, but you'd never be able to do that online." Maybe, maybe not (I dunno, I never tried), but I wouldn't want to be the guy trying to return them, not when I'm accurately hitting 120+ serves out wide or down the line whenever I want. They'd be at the server's mercy on every point. I mean, you can only do so much to return something like that, like guess and move before contact.
Like I said in an earlier post, my advice is that you master, and play only with, the button scheme, because, by going down the path of advanced serving, you'll initially love the results but soon grow tired of the ease wherewith you achieve them. It's a very fun game except for that. I personally wish I had never discovered advanced serving.