I rarely iron duvet covers or fitted sheets.In fact I never iron fitted sheets.Most of this stuff is poly/cotton anyway. So..if you give them a spin on the lowest spin cycle and hang them on the line straight away they should blow out nice and crease free. If I do iron them(when I am feeling virtuous)I fold them in half lengthways iron down one side,turn then iron the other side the then fold them so the unironed side is outwards and reverse the procedure.Although when you put them on the bed you will have a crease down the middle.
Or straight out of the machine and into the drier ..straight out again when dry and fold .If you leave them in the drier they will crease.
Also Dylon fabric care Easy Iron is great for creases and makes your washing smell lovely but not stiff and starchy.