I got a rather low 2:1 in my first year - I had some atrocious results in my first semester whilst recovering from a viral disease and my work ethic was a little lax even afterwards. Before starting my second year, I read all the feedback I'd received in year one, figured out where I was going wrong in exams and coursework, spoke extensively with my tutors and my adviser and forced myself to attend all lectures and seminars despite picking up a serious leg injury by accident shortly after term began.
My marks picked up and I started to receive many firsts and high 2:1s in course assessments. I was a few marks away from receiving a first for the year but was still extremely pleased with the progress I'd made. My results gave me the confidence to aim for a first overall and it motivated me to once again go over my work and figure out where I was haemorrhaging marks. I used my summer holidays that year to get in touch with my third year tutors so I could undertake some preparatory work over the summer. I started the year determined to gain a first.
I worked extremely hard throughout my third year, including many days and nights at my university library, and received a first in every assessment (bar one essay) and thus gained a first-class honours degree. If there's a 'take-home message' in all of this, I guess that it's not to be complacent. Don't assume that you can jump two degree classifications overnight. It times time, practice and effort and the sooner you can modify your work-habits and learn from your mistakes, the easier it'll be in the long run to gain those higher marks. Hope that helps!