Heh. It comes of reading a lot of books and knowing the basics of writing. Taking some time to build up suspense (even if it's just a little) can really make the eventual payoff that much better.
And the best part is that you can mix this in with just minor worries. For instance, after being hit by a spell, you start coughing every once in a while, and begin sweating uncontrollably. Are you merely down with the flu? Or is it something more... sinister? By having the relevant rolls hidden, you can really keep your players on their toes.
Also, it's worth noting that this is only really effective if there's both minor problems and major problems. If there's someone transforming every turn, players will start to expect it, and it'll lose the shock value. On the other hand, if you never toss something huge their way, they'll quit being worried about the minor problems you do give them. However, if you manage to keep a good balance, giving them the occasional major thing where it makes sense, that's when you'll get the best results.
@Tarran
Honestly, just open a word document and write it down. If you're planning on a more in-depth RTD, you'll probably be writing stuff down anyway. I mean, do you think Piecewise runs Einstenian Roulette entirely from memory?
Also, you're thinking a bit one-dimensional. You could have it be a magic curse that the players need to rush to break, compared to standard polymorphing's temporary effects. And then instead of fully breaking it, the players could try to find a counterspell to block the less desirable effects while keeping the benefits intact.
It's all in how you write it, really. And it's worth noting that the end result doesn't have to be horrible (I mean, becoming a dragon does has its upsides), and if it's known to be from a benign source, you could easily go with gentler descriptions. That said, since most of these transformations are probably going to be inflicted by an enemy (or spell misfire), you'll usually be aiming for maximum paranoia.