I always string up my tomatoes. To find a point to anchor the string. I cut a few strips( 5cm x 1cm) of metal from scarps in my shed drilled to holes in them, bent them half way along and fitted them under some of the frame nuts and bolts. this gave me some horizontal lines across the green house to tie the vertical supports too. A loop round the base of the tomato taking a turn round the plant every 20cm gives them support. This idea I picked up from Island Tomato's open day. To encourage growth they cut off all the green leafs below the tomato bunches when the tomato's start to grow. This way I can get up to 20 plants in my 6' x 4' green house.