PHEW. Just read through the entirety of this thread!
I have shrinkied more dinks in my life than I care to admit, both hand-drawn and printed stuff. I've done a lot just for myself, but I also used them to make Christmas ornaments once for a hockey team and then the same year also used them as that year's craft at our annual Christmas cookie party. (Wayyyy too much shrinking going on that year!

) I've had pretty much every problem imaginable at one point or another, and even after all the times I've tried it I can't perfect the art so I figured I should come in here and try to pick up some tips.
One thing I didn't see mentioned in here is putting something on top while baking to keep them flat. When I do small things they seem to work pretty well (they curl and then uncurl like they should) but bigger things, especially things that are long and skinny, tend to curl so much that the ends meet and stick. In my experience, once it sticks to itself it's game over for that particular piece. I used to do the whole parchment-paper-on-top thing like the instructions said, but even that couldn't prevent the sticking, so I started throwing an empty baking pan on top. I just plunk one of my 13x9s down on top since it's nice and heavy and voila - no curling! I can't promise that it wouldn't affect the shrinking, though; it might have some effect on the "warping" issue.
As far as the printable stuff, I saw some people asking how to tell what side to print on. I've always been able to do the whole wet-a-finger-and-touch-both-sides process, but after doing that once I noticed I can also SEE the difference. And feel it, actually. The printable side is coated with something that can sort of "absorb" the ink so it doesn't just bead up, so it's not quite as shiny. I don't know if that will help anybody, but it's worth a shot. And printing on the other side definitely does NOT work, so don't even think about it!

And for anyone wondering how much to lighten their pictures by, well, it's the same as the ratio of shrinking. If your plastic was going to end up 1/3 its original size and you originally printed it normally, when it shrunk it would have 3 times as much ink on it as it should, and it'd be 3 times darker, basically. So if it WAS shrinking to 1/3 of the size, you would want to lower to opacity of the image to 33%. If it's ending up 40% of the size, the opacity should be 40%. Basically - obviously you may still need to do some trial and error because there are always variables you can't account for, but if you think about it it makes sense and gives you a good guideline to start with!
I was so surprised reading through this thread to see people referring to a rough side and a smooth side - a lot of people are talking like that's the norm, but all the plastics I've used are smooth on both sides. I *wish* I had a rough side - it might make it easier! I definitely want to try sanding and using colored pencils to see if that helps at all.
My main issue is getting a nice even color. Everything I've done has been either printed or Sharpied. The printed ones end up with this super matte finish that isn't exactly what I'd like. The Sharpied ones end up with the color being very mottled and uneven - you can see the "lines" made in whatever direction I was coloring in, and it even gets darker in the spots where my Sharpie-strokes overlapped more. Plus using Sharpies gives me a very limited range of colors, especially since anything remotely dark-colored ends up way too dark after shrinking - I'm limited to the lighter Sharpie colors. Do you guys have better luck with sanding and using colored pencils to get smooth, even color?
Also, bad news - Michaels doesn't carry shrink plastic anymore. Or at least, the Michaels near me doesn't. I was in there a week or two ago buying a bunch of stuff because I got some funding for my Etsy shop so I was having fun spending money that wasn't mine (

) and I wanted to pick up some extra shrink plastic, but I couldn't find it where I expected to. I looked all over the store and eventually asked someone where it was, got handed off to other employees a couple times, followed one poor girl all over while she tried to figure out where it was, and eventually she called somebody else and determined that they don't carry it anymore. Sad times!
And now I'm pretty sure this has become a ridiculously long post, but that's everything I have to say on the subject (for the time being!).

Edit: WOWW, when I said "ridiculously long" I really wasn't kidding! Sorry all! I applaud anyone who actually bothers to read all that...or...any of it.