Make sure you're saving in a format that doesn't compromise the image quality for printing (like PNG and PSD). Also make sure you also increase the megapixels in PS Touch's settings. (At least 3 megapixels for 8 x 10.)
It's beause all the files saved on PS Touch are natively 72dpi. To try to compensate for that I would do what Warunicon suggested. Just in case your unclear how to increase the pixels it's in the preference's menu under maximum import resolution. Honestly I would just set it to the highest setting the your device can handle, mine is set to the maximum(Galaxy Note 10.1). The limit is smaller if you have PS Touch version 1.2 so I would update to version 1.3 if you haven't.
As far I know all the image editing mobile apps are also 72dpi. I'm assuming it's to keep the file size down because of the limits on hardware. I don't print from the files I create on my tablet and only use it as a starting point for my paintings in Photoshop so it's not an issue for me. Maybe someone else who use it for photo editing can point you to an app that has a higher or adjustable dpi, but increasing the maximum file size should be enough.