Sometimes, to write good poetry, you have to stop thinking about writing good poetry. I mean it! Trying to write good poetry is like trying to fit the glass slipper that wasn't made for your foot on; it just isn't going to happen.
So accept that, and move on. But how can you do that? Well, with a little focus on not trying to create your best poem ever each time you sit down, it is possible! Try these tips, which are what I usually try to do.
1. Don't Censor Yourself
If you find yourself starting to right something and immediately your conscious mind fights back and says "you don't want anyone knowing that / reading that / etc," turn that thought off and keep moving! Your first draft should be for you and only you. You just need to get the words down as they flow into your mind--don't overthink it, don't censor them, and don't second guess yourself. You have time to edit later.
2. Let Yourself Be Messy
Every word isn't going to fit into your first draft, so just scribble it in anyways, take a few deep breaths and move on. Be open to any thoughts that come into your mind, and follow them even if the result isn't so pretty. You can't control this step: you've gotta let it go.
3. Usually, Set It Down
After getting the base of your poem down or exhausting whatever is flowing in your head, set it down for a few hours, days, weeks or more. Leave it be. Let it settle. When you pick it up with fresh eyes, you might change everything about it, but you'll be making good progress.
There are, however, some times when you'll want to revise it right away, but you'll know when that is, because you won't be able to get your mind off its words until you touch and recreate them some more. Don't worry about doing this: you can always then sit down the revised poem and pick it up again later.
4. Edit later!
Pick it up for editing later, as opposed to working on editing while you're still generating the base of your piece. Your generative creative state and often reductive editing state should be separated so that you can accomplish both tasks well.