Hi Mike,
A Teflon sheet or just a sheet of butchers' paper is a popular way to protect the shirts.
Usual temp. for dye sublimation is around 400 F or 200C. Pressure should be light, time - about 45-60 sec.
There are quite a few tips on avoiding press lines in previous threads - you may want to do a search in dye-sub section. Here are some:
Check your pressure - it should be light.
Use an insert (like Vapor foam pillows) in between the layers of the shirt to elevate printable area.
Make sure the transfer paper over-hangs the insert - it will eliminate paper lines
Lint roller is a very useful tool to have - to clean the printable area from any specks of threads, dust and other contamination.
Good luck
