How old are you Lix? Age plays a large factor in how much sleep your body requires. When I was a teenager, I needed to get a minimum of 10 hour a night, but 12-14 was preferable. Now I need more like about 8-12, depending on how active I was throughout the day, and other factors.
You may be fighting a battle that can't be won. I used to try with all my might to do the same thing as you, but it always ended very ugly. If your body needs 12 hours of sleep, then it needs 12 hours of sleep. You may do better to just accept this fact and learn to live with it. One thing to keep in mind is that if you force yourself up early one day, then it will just be that much harder to get up the next day, because your bidy will try to make up for the amount of sleep that was lost.
One thing to try would be to slowly shave off small increments of sleep over a period of time. Maybe start with setting your alarm to go off 11.5 hours after you go to sleep, then after a week or two of that set it to 11 hours and so on. Shaving off about a half hour each couple of weeks. Also allow yourself one day per week to sleep in as late as your body desires. I use my sunday as this day, and it is not unikely for me to sleep for 12-15 hours! But this makes it easier to loose a couple hours of sleep throughout the rest of the week.
Anyway, thats all I got for now.