I've spent the morning editing and commenting on cover letters and portfolio drafts. Here's three pieces of advice I used repeatedly:
1. "Use specific examples to develop your discussions and claims."
2. "Contextualize the examples you provide by discussing in the text of your letter/portfolio why your example is significant."
3. "Focus your discussion on what you have learned and not on the effort you put in to learn."
Let me develop the last of the three a tad more. Effort has some impact on your grade in every aspect of life, but in college, professors are much more concerned with what you have learned, your expertise, and the demonstration of your skills than they are in how hard you've worked. If all goes well, work in college (and in life) is demanding, challenging, difficult, and engaging. The same is true of that you'll find in most careers worth the work you put in to keep them. You do the work which is necessary to succeed. It's expected, and it usually receives very little praise. Usually, you get kodus for being productive, not for how hard you've worked to be productive.
Ben Franklin once said, if you want to get praised for how hard you work, you must be seen to work. This is why he made sure he was seen delivering his own papers, that is, so folks would notice, and his ethos (his reputation) would be raised as a hard worker. His delievering the papers, however, would have been meaningless if he didn't have the papers to deliever.