I normally start my day with coffee and email, and I review the tasks I'm currently working on and my appointments for that day. I also try to do some smaller tasks first to get me going.
If I have a large task like you mentioned I break it up - breaking a large task up into smaller achievable bits really helps with motivation. I currently have a load of qualitative data to analyse, so I have broken it up into 30 chunks: 10 participants x 3 bits of analysis for each. I have a list of these 30 chunks, and when I complete one I tick it off my list with a MASSIVE tick and reward myself with coffee or chocolate, or even with 20 mins relaxation or reading a magazine.
Working in different places often helps too - e.g. I go to the office and do one chunk of work, have a coffee and do another chunk of work in the library, walk home and have lunch then do another chunk of work while lying on the sofa, etc.