I read this post:
http://www.t-shirtforums.com/p382930-post7.html
And then looked in the quilting section at walmart. I found an 8x8 ruler, by the same company, that has a 4.5x4.5 square cut out of the center. It's been awesome...
Here's how I center my shirts:
1) I pull them over the platen and use the side seams (IF they have them) to help center the shirt, in addition to the press line you mention.
2) I then take one armpit seam in each hand and see if it "feels" like it's even (usually hangs off the side with most standard sizes).
3) Then I look and make sure the neck appears centered.
4) I then take my SUPER cool new ruler and set it on the shirt, on the platen. I match the top of the ruler with the end of the platen (I put my neckline on the end of the platen towards me -- so I'm pressing "upside down"). And then use it to put my design on.
Now, most of my designs are quite small, but I know this ruler would still come in handy. Sometimes I put the design on freehand and then lay the ruler over and see how I did. It has a grid on it, and you can see through it; so you can use the lines to see how square your design is...
Hope this helps a little. They had a variety of rulers that I think would be useful. In reading all the "alignment" threads here, I realized (and now agree) that it's a matter of how much time you want to to take per shirt. And you get good at it after a while. Do you want to sit there laying out rulers and guides, taking 5 mins just to line things up? There's a post on here about someone who did 20 or however many shirts -- half free hand, half spending time with rulers and guides. S/he then looked them over and the freehand ones came out the best aligned overall...
If you haven't already, order the free sample kits that all of the transfer folks offer. And then you can use them to practice with on some cull shirts...
Good luck!
-Kristie
Need help with Image/design centering for small 9in by 12in heat press 
