Make a simple practice barracade, take it to a grassy park and set it up on a short loop so you can ride and practice dismounts/mounts. You might also practice turning round a hairpin corner, cornering an off-camber hillside turn and carrying your bike on run-ups.
Good advice from GrayJay.
If you have access to a Mountain bike I would recommend spending some hours riding some twisty, fairly-steep downhill singletrack to get used to maneuvering on loose surfaces. Many folks who come to cyclocross from a background of primarily road riding spend a season or two improving their bike handling.