Firstly, sorry for long post....Don't do billions of crunches, ruins your posture, puts lots of strain on your back, forces your stomach muscles into an awkward shape and does nothing for the deeper muscles. Oh and it will bore you WITLESS.
You need to think as much as possible about bringing your tummy button in towards your spine, without lifting shoulders, try and do it as much as possible during the day. Try and 'brace' with your abs when you're lifting anything heavy (nearly one year olds for example!). Try and locate your transverse abdominus which is the deeper muscle which goes across the lower section of your abs which gives real strength to your middle esp post babies - if you put your finger tips on your lower tummy just on the inside of your hip bones and cough you should feel a muscle press against your fingers. This is the muscle you're trying to use more.
The plank is a brilliant exercise, but quite tough - you need to build up to it. Can explain it if you don't know which one this is but don't want to teach grandmother to suck eggs and all that. Also exercises which use your abs without making you crunch over e.g. sit with knees bent feet flat on floor, place hands on knees, sit up tall as you can. begin to lean back, keeping hands near knees and trying as hard as poss to keep your tummy button in towards your spine. lean back, until you feel you can just hold the position and hold for a count of 3 before sitting up really tall again.
Finally, straight leg head raises (!) work really well esp post babies - so lie on your back with straight legs and toes slightly pointed. Try to lift your head and shoulders up off the floor, look towards your toes. Try keeping your shoulders up off the floor but do small raises (pulses) looking towards your toes the whole time. Hands best kept over your hip bones/thighs rather than behind your head.
Pilates is great, works mainly on the principle of tummy button in towards spine as much as possible and using deep abdominals, agree with notquitecockney - make sure you find a good teacher. Independent pilates teachers can be better than ones in gyms/leisure centres as they often have more qualifications (not always). You can 'qualify' as a 'mat' pilates teacher in a weekend, which just isn't enough to really know what you're on about! You should look for someone who has more than just a 'mat' pilates qualification....
Do your pelvic floor every day without fail and you should do enough squeezes so you think you can't do any more, and try and make sure you 'squeeze' without clenching your butt and your thighs!
Have billions of abs exercises,happy to bang on at length about them all. Pet subject, can't you tell!
Oh and lots of orgasms really helps with your pelvic tone (apparently)