Habits Can Prevent Improvement

I had the habit of getting tea at work each day. I'd fill up my cup to the top, but the tea would always get cold and I'd have to spill it out in my garbage can. This worried me because the cleaning staff would have to deal with this.

One day it occurred to me that I shouldn't fill the cup up all the way. I should fill it up enough so that I don't have to pour any out.  

Now in hindsight it seems obvious that I should have done this but at the time it wasn't. What habits do you have that are preventing you from doing something more optimally?