Writing is hard enough. But somehow we find ways to make it still harder.
If you're struggling and you become deeply frustrated during a writing session, don't make it harder on yourself by keeping at it. Don't force things. Put the work up for a while. Go for a long run. Read a diverting book or go see a classic movie. Have a beer. Spend time with friends.
Leaving a writing project until the last minute also makes writing harder than it needs to be. That's why it's so important to keep some work in the can and why it's important to set an effective writing schedule.
When the piece you're working on is muddled and getting more muddled with every edit, don't make it harder by muddling your work still more. Put it up for a day or two, and then tackle it again. You'll find the right words and the right voice.
When your mind is foggy, don't make it harder by forcing yourself to work on cognitively demanding writing tasks. And when your mind is clear, don't squander that clarity on procrastination, web-surfing, or other wasteful activities.
Most importantly, when you're having a really good day writing, keep going. Don't waste those precious moments of productivity. Use them for all they are worth.