2 things you need to make it comfy - support and cushioning.
For us - we keep 2 big pieces of ply wood just for our sofa bed - DH will unfold it - take the mattress off - put the two pieces of wood over the frame then put the mattress on. Since sofa mattress' are so dang thin - this eliminated the feeling of the support bar in the middle etc.
In addition we have a big foam pad that we place over the mattress. This helps with cushioning but also with keeping the sheets on since it gives the mattress more height like a real one.