Thanks to the excellent information in this site, I have successfully created three t-shirt designs. I was able to print them and transfer them without any problems. Here are a few things I learned that have yet to be covered in this thread.
I used transfers from Michaels (Wilton EasyImage transfers) and Avery, both for dark fabrics. We prewashed our fabric, without fabric softener, removed them immediately from the washer and line-dried overnight.
1) Prices for the dark fabric transfers are more expensive than transfers for light fabrics. You also receive less transfers but they appear to have a better quality. We did three shirts. One was black, one was green (almost lime green) and the other was a baby blue, utilizing a pink background design which came out beautifully. Therefore, I think the dark fabric transfers could probably be used with any color fabric and the results will be of a high quality.
2) We preferred the Avery transfers, again due to quality. Prices were comparable.
3) None of the instructions mention what the printer paper settings should be. After a little experimentation, we used the following setting: photo quality paper - semi gloss and used the highest ink setting (in our case, photo.)
4) The above settings not only helped the color saturation quality but really helped the printer 'pull' the paper into the printer. Using other settings caused the printer to think the paper was thicker than it was and crinkled the edges of the transfer, causing all sorts of wonderful issues.
5) We let the transfers 'dry' for a couple of hours before doing the pressing. It was a step just in case any of the printer ink had to dry. Might not be necessary but it didn't hurt.
6) Instead of using an iron, we used what might be considered a commercial press for our transfers. It worked perfectly and seamlessly. We held the press for 3 minutes.
Update: Upon a second look, we thought our transfers were more of a matte finish that we had imagined them to be. We decided to put a sheet of parchment back over the completed transfer and apply some more heat via an iron. Great decision, because the transfers looked even better than before! My guess is we didn't set the heat up high enough on the commercial press setting.
7) We have not done a vinegar wash after the transfer. We are still looking into that and might do a test wash with and without to see the best results. I'll post again if I discover anything.
Update: Based upon further research using Google, it appears that a cup of vinegar is the best way to 'set' the colors so they do not run and remain vivid. Here is one of many articles I found: Fabrics