One drill we do for our roundoffs is to do them over something. The simplest is to use a panel mat, as it's the most universal mat to have on hand. Fold the panel mat so 2 of the panels form an upside down V, place the hands in front of the V and land with the feet behind without knocking the mat over. We also do it over octagons etc.
Also try having contests to see who can do the longest roundoff- it's rather hard to make your roundoff long if your chest stays down too long.
In general, my coach always tells us that our hurdles should be long and low, rather than upright, for the best power from our roundoffs. Working on the hurdles can anly help, as well
~Katy