Maybe this is obvious, but I've only recently been using it a lot. This example was used for a cover up, but I use it even more when I'm planning a tattoo for a certain space.

All I do is hold up a ruler to the piece when I take a picture. That's it.

After transferring the images to my computer, I open up Photoshop.
Create a new file (File > New). For a new document I always use the Photoshop Preset: U.S. Paper. (Size - Letter) This creates a standard 8.5" by 11" sheet at a 300 dots per inch resolution.

Next, I open the image I want and drag it onto the new document. Making sure the rulers are showing (View - Rulers) I click on the ruler and drag/pull a guide down to the inch mark. And I drag another down to the two inch mark.

Then it's a matter of resizing the photo; enlarging or reducing enough so the inch and two inch marks line up with the guides.

Pretty easy.

Then I have my size. Sometimes I print it out and draw over it. Other times I use my Wacom and sketch right there. It's perfect. And it helps with figuring out sizing for cover ups.

I also have paper rulers I print out and use as well. I like using a new one for every person, so I just print them out and throw them away. It's nothing special, but, when I get time, I'll throw it in the reference section so it's easy to just grab and print.

It took awhile, but I've just posted some tattoos in a new section of the gallery: Continuation. I'll continue to post new stuff in this section for the next couple of months.

Also, I've added a gallery of photos I take and send from my cell: iPhone Photos. I'm still posting to Flickr, but this area is a little more tattoo and experience specific.

It looks like I'll be tattooing the Baltimore Tattoo Convention next month. I've yet to tattoo outside of my locale, so I'm excited to jump in.

I've open availability if you'll be around the area - It's Jan 23rd through the 25th.
Connect with me through email: david@allentattoo.com

More info here.