Dismounts: start by unclipping your LEFT foot, and placing it on the pedal. This means you don't have to worry about getting unclipped in a hurry. THEN, unclip your right foot, and swing your leg over the saddle and behind. Coast with the left foot resting on the left pedal, and hop off just before the barrier/obstacle.

Remounts: here's the mantra I use to teach this - Step (left), Step (right), Step (left) and over. Do it at a WALKING pace to start with. On the third step, make sure that you push off on that step, and there is NO NEED TO JUMP onto the bike - slide the right leg over, getting the saddle to hit the THIGH as you make contact with the bike. Don't worry about getting clipped in right away - feet on the pedals, and you will get that as you start pedaling. The other key here is that on the Step, Step, Step - your bike should be in front of you so that when you push off and do the OVER portion, you are catching up to your bike vs waiting for it.

This is much easier to demonstrate, but it's the simplest explanation possible.

SheFly

p.s.
If you are going to be racing with clipless pedals, do not practice on flats. You need to get used to the clipping out on the dismount, and clipping in on the remount.