Think most points covered so far in the replies, I also have an Astra, and although, I could say that during wet days when I'm out delivering, it hasnt got the best de-misting system, I've not had major problems with in, and never any problems when starting off in the morning.
1, Make sure you've not got any water leaks in the car, cause as soon as you get in, it'll start misting up
2, make sure the windsreen and windows are super clean on the inside, possibly try one of the 'ant-mist' products from the car accessories shop
3, try not to wipe the mist off, as you'll likely be adding allsorts onto the glass, that'l make it mist even more next time
4, turn off the re-circulate switch, hit de-mist, and close all of the dash air vents while the windscreen de-mist (then turn on the side vents and direct at the side windows
If you've got no water leaks in the car, your windows are clean, and the vent that points at the windscreen isnt blocked, then you can't have any problems !
P.S - I'll share with you a little nightmare I once had, regarding a misting windscreen. I used to do a little car valeting, and did the bosses Lexus, all the windows were perfectly polished and clean inside and out. We had to go on a business trip the same day. I was to drive, now this car had one of those computer heating systems, with a few too many buttons and modes, we'd been going for about 10 minutes and were on a 50mph road, when the window suddenly started to mist from the bottom, I glanced at the heater controls for a couple of seconds, and made my selection...within the next 5 or six seconds the mist rose to cover the whole of the inside of the window, reducing visability to absolute zero. I could not even see where the road was, and I was still doing at least 45mph. I had no option to guess where to steer and hit the brakes. Shortly after pulling up, the window cleared and we were on our way. It was one of the most scary driving experiences of my life !
Paul