Recently I had a customer ask the secret to not messing up cards. She found that as she was writing letters on her custom stationery, she was writing something that sounded off, spelled something wrong, or something didn't quite fit on the line as she was hoping. I'm sure this is familiar territory to all of us (re: we've all been there... do we draw a heart over the misspelling and call it good, own the squishy sentence that in no-way should have fit on that line, or do we scrap everything and start all over?).
My tip (and what I practice at home, because I'm totally guilty of drawing hearts over misspellings* and scrapping everything and starting all over): Write your letter out on a similar sized scratch piece of paper before using the card or custom stationery. I sometimes start with typing a draft email if it's a more complex message where I want the wording to be just so. Regardless, I do make it a habit to write the note out by hand beforehand.
I find it helps with spacing things out evenly. It helps me better articulate what I'm trying to communicate. And it warms up my writing muscles so I'm putting my best handwriting to use.
*When I was just a wee-little-girl, my mom taught me to "just draw a heart" over any misspellings. I'm pretty sure all my aunts and uncles, grandparents and friends received letters from me with random hearts drawn mid-sentence. It's charming, and a really great way to encourage children to write letters and thank yous.