While this may sound almost silly, I actually find it easier to get the essays moving once I feel I've made some progress on the overall application - that is, get all the online stuff done in your first sitting. Fill in all the sections, upload your resume, transcript etc. It actually helps me transition into a new application without taxing myself too much, and then you feel as if you've already achieved a fair bit even before you sit down to start your first essay. The essays become easier for me when they are the sole focus and all the other irritating stuff is out of the way.
Next, first attempt an essay that's easily taken from one you've already done - so for HBS, my advice would be to tackle the career goals essay first, assuming you're doing that. It's definitely easier to write the second essay knowing you've got a pretty solid / tried-and-tested draft of the first essay behind you.
All this may seem obvious, but I think it does help one's overall attitude towards an application.