Right. I've had diastasis for years. I am now at maintenance stage where a few exercises each day keep the separation closed.
Before saying anything else i must say, STOP doing 30DS ab workouts, stop doing crunches, planks and anything 6 pack related. These exercises all put pressure on the separation and make it worse!!
The first excercise you need to do is isolate the transverse abdominus. This is the big corset like muscle which holds everything in from the inside. Strenthening this muscle will heal your separation.
Stand up feet shoulder width apart and leave arms by your sides. Focus on your belly button for a moment and then pull your belly button in. You want to imagine there is a string tied from the inside of your belly button, to your spine and out a hole in your back and that it is being pulled from behind you. It should feel tight, possibly in your back as well as front. You shouldn't be bending your spine or dragging your whole stomach in, breathe in and out through it and hold it as long as you can. That is your transverse muscle.
Exercises to do to heal separation
As above draw your transverse muscle in. This time pull your pelvic floor in at the same time. Now place a finger in your belly button and still holding the contraction of muscle pulse it. You should aim to keep it as tight as possible while pulsing. Keep going for as long as you can.
Imagine your tummy is a lift. Contract the transverse muscle in stages as though you are going up floors in a lift. Once you've got used to this take the lift up to each floor and down again in between. Aim to work right up to the sixth floor at your belly button.
Pelvic tilts. Lie on floor, knees bent, hands on tummy. As you exhale tilt your pelvic bone up and roll it slightly. Your tummy should hollow not bulge.
Head raises. Lie in same position. Lift your head and nod your chin to chest and down again. Repeat. Don't come up into a crunch, keep lower half of ahoulder blades on the floor.
The key thing to focus on is your alignment as you go about your day. Try and be tummy aware. When you are standing or walking try and adjust your posture, straighten spine, shoulders back, pull in your transverse muscle a little way. If you are about to bed down contract the transverse muscle first and hold it as you bend and straighten. When you lift your child/something heavy contract the transverse, straighten spine and pull in pelvic floor. If you spend a few days being really conscious of your posture and your tummy you will start to automatically adjust posture, pull that belly button to spine string.
Diastasis means there is only connective tissue holding the muscles together and your organs in. It doesn't matter how many of the exercises you do if the rest of the day you are letting your belly push out, you are leaning and lifting without contracting it which puts pressure on the separation and pushes it further apart.
Like pelvic floor exercises you can contract the transverse muscle without it being obvious and while sitting, standing or walking. So try and do it as often as you can through the day. Pull it in and hold it for as long as possible. Pulse it. Lean slightly to each side contracting it as you lean. Keep strengthening it and it will begin to pull everything back in and close the separation.
If you want to spend a bit there's two schools of healing you can google. Wendy Powell has created MuTu. Quite pricey but does diet plans and work outs as well as heal diastasis. There's also The Tupler Technique.
Hope that helps.