A lot of good tips in here. Also make sure you drink enough and let in a fresh breeze of air every now and then.
I was having major concentration problems doing programming work and staying focused. Opening the windows in regular intervals - like once every 2 hours for 15 minutes - has done wonders for that.
Depending on the activity (studying in your case) listening to music can help, id suggest some ambient chill music - without beat if possible. But i think that varies from person to person
