I can eyeball it and get it pretty close. On those occasions when I am noticeably off-center, I add some stickers, bling, or some other kind of art to the side with the most space. Put it at an angle. Then, you can also add smaller stickers, bling, etc to the other side, but often you will not have to. You will learn how to cover your small mistakes.
I am going to look in my gallery and find an example for you.
This is an example of a LO of mine where the letters are not centered. If you look, you can see that for the word "grandma" at the top, I have more room on the right hand side. I put some buttons there, diagonally, and I balanced it on the other side with the butterfly placed at an angle. I don't think it's noticeable that my letters are not perfectly centered.
Hope that helps. Like I said, you will learn how to cover your boo-boos.
Get a centering ruler! Love mine. The 0 is in the middle so you can line stuff up easier. I stick the very edge of my letters to the ruler and then when they're where I want, I push the tops down and remove the ruler from the bottoms then push those down.
I linked a few from the store here. I found mine at J's in the quilting section.
Centering rulers are fantastic! However, I try not to center things very much because the eye will detect the smallest flaws, and I know I'm very flawed when it comes to measureing and precision! I try to do asymmetrical whenever possible.
I am bad at measuring, too. Tried to use a ruler once, and my hand kept moving it as I tried to do stuff. That is why I eyeball stuff now.
In my recent J's order I bought a magnetic cutting mat. Hopefully it solves the problem of MRS (Moving Ruler Syndrome). I haven't had a chance to use it yet, but playing around with it, it looks like it will be a big help. It had better help or I'm going to have to grow a third hand.
I have accepted the fact that I am "not centered" so I plan to be asymetrical or put letters in a curved line. That helps me accept the fact that I can't center perfectly. The best tip someone gave me is to figure out what letter is in the middle of the word... then place it in the middle of the line. Place one letter to the left, one to the right, etc. until you have all the letters placed.
I don't do any of that stuff - tried, and was rubbish at it. Now I just pick the middle letter in the word and stick that on in the centre of the space I have. The other letters get placed lightly around it (left first, then right, then left etc.) until I'm happy with the placement. Then I stick them down properly.
Or, you can stick each of your alphas onto mats. Then you can move them around until you're happy with the positioning before you stick them permanently. Good luck!
Oh, I love the idea with the Post-its. I use my clear Fiskars ruler, but it can get unwieldy.
Sometimes I work from the center letter outwards, but mostly only as a guideline - almost every font I've used has, for example, wide Ms and Ws, but narrow Is. More usually, with stickers, I stick them on the edge of a leftover empty sticker sheet and measure them to see where the center is, and work from there, instead of guessing that it's the center letter. Helps with letters of different widths and also with spaces between words. With die-cut letters I just lay them on the ruler and estimate.
I also use a clear ruler. I place just the bottom portion of the sticker right on the ruler and then move it around on the page until they are right where I want them. Then I press down on the top part of the stickers and gently pull the ruler off the bottom.
I have the both titletopias from Creative Memories. They help you place letters straight, sideways,upside down, wavy like a banner,circle or spiral. When I place my letteres on the template, I can tell exactly how much room I need for the word and can easily center.
You also can place your letters on a ruler edge or piece of sticker paper to line up then lift.
Press the letters down really well and then remove the ruler from each letter individually by peeling the lIttle bit that's stuck to it off. Once all the letters are free, slide the ruler out and press the rest of the letter down. Easy peasy.