long and low work is your best bet without hills working using correct muscles too rather than just poldding along, oh and trotting in long grass to get them to pick up their feet/stretch to look etc i find is also good, carrot stretches or strapping at home too can help
i do all my work on hacks (no arena) and lunging in the field - and lunging over raised trotting poles - i've managed to build topline that way
Carrot stretches are excellent at getting horses to let go and relax over their backs As p_h has said, lots of long and low work, make sure they are always working into the bridle (strong walk!) and vary the outline too, so go from long and low to a deeper outline, then up in the neck. Shoulder in, leg yield etc in the varying outlines will help