I have been dye-subbing shirts for over 5 years on daily bases and here's what works best for me:
1. Place t-shirt on the platen
2. Lint-roll it thoroughly
3. Put an insert* between front and back layers of the shirt - the insert should be slightly bigger than the graphics, but smaller then the transfer paper.
4. Position the transfer - make sure it's big enough for transfer paper edges to over-hang the insert.
5. Press on very light pressure with the combination of time and temperature suitable for a particular fabric type you are working with.
* As an insert I use heat resistant foam pillows with Teflon "pillowcases". I make them myself and have a collection in different sizes and shapes to cover most standard jobs. Teflon cover/”pillowcase” makes it easier to slip an insert in and out of the shirts and helps to keep an insert clean as it's easy to wipeoff if any ink goes through the fabric.
If the design is rather big so the transfer paper is bigger than the platen size itself, it may not be practical to elevate the pressing area as there would be no paper lines anyway.
If you can see heat press platen impression on the t-shirt, the pressure could be a little too high.
Unfortunately, there are always going to be fabrics/garments that behave differently from others – some of them don’t bounce back very well after pressing, especially if they are heavily textured i.e. ribbed, waffle weaved etc. so more effort and experimenting is required to get the best result.