18lb lost still so far to go !!

I started my diet on 1st Jan 2012 and since then have been trying really hard to keep motivated. I do a bit of exercise as and when I can, the only thing that seems to stop me is time and a niggling back/hip problem. I have a total of 4 stone (56 lbs) to loose and have lost 18lbs so far so still have 38 to go. Any ideas of speeding up the weight loss or at least ways of keeping motivated ?