When using spray starch - you’re right to use it sparingly, but what’s more - use it on the reverse side of the fabric, that way any marks left from the starch will not be seen. In other words, turn the sleeves inside out to spray then, then turn the sleeves back round re right way.
As for trousers… always makes sure that the door seams line up - that should ensure the front and rear creases in the legs are in the correct place.
Press the creases on place, then turn the rousers inside out.
Take a piece of soap (I use the small ones they give away in hotels - note do not use liquid soaps!) and run the edge of the soap down the inside of the crease… it will leave a trail a bit like a slug.
Turn the trousers back round the right way and iron again.
The soap melts into the fabric and gives a sharper crease. Again use sparingly at first.