Re: sublimation help
As Debz said - using a teflon pillow or foam insert that is slightly smaller than your transfer will help to eliminate paper lines as the transfer edges will be hanging over the pillow/insert and will not get pressed.
If your box is not from paper lines, but from heat press upper platen - try dropping down the pressure. Generally, with most fabrics light to medium pressure should give you good results. You may need to increase your pressing time to compensate for lower pressure. I usually do the shirts for 60 sec at 200C.
If you need true colour representation of your designs (i.e. logos, sports club colours, particular Pantone colours you are trying to match) - it's better to stick with white shirts.
You will get very good results of light coloured pastels, but as Charles mentioned - there will be a slight colour shift: blue print on yellow shirt will have green-ish hue, red colour on yellow shirt will have orange hue etc.
On darker shirts like red, orange, blue you can still print with dye-sub, but only very dark coloured designs and black prints will stand out.