The first cover letter I wrote was to my #1 site at this point. Since my interests matched up so well it basically wrote itself with only 2-3 revisions. The rest are requiring a bit more effort from me, but its nice to have that first good one to draw from as a template.
Another rule I go by is that if I dont have the motivation to write, I generally dont. I think we have plenty of time until essays are due so I usually take a solid break if I am unmotivated, do something else, and come back to it refreshed. I've found that some of my best ideas will come to me when I'm doing something else like working out or reading for fun. Thats the right hemisphere for you...never works well under pressure.