Good girl!
Well I suppose I made a concious effort to be fitter, even when I had morning sickness (I lay down and died last time and lost fitness), although I didn't get the chance not to be wih a toddler. I also trained him to get in and out of the car/highchair/cot/bath with either just my hand to support, or by me lifting from sitting, but without me picking him up whilst standing.
Because I knew all the harmful positions and movements from last time I didn't do them, and because I'm a bit over flexible I have taken care not to stretch myself open too much. Pelvic Partnership do a good leaflet on these.
Mainly the core/pelvic/back strengthening and Osteopathy though I think is what helped. Hey, it could have been just destined not to happen this time anyway, but I really don't think I'm that lucky!
I was like you last time, didn't get diagnosed until it was too late, no-one I saw knew much, apart from an obstetric physio who then told me no excercise as I'd gone past the point and everything was too inflamed and sore. Even short dog walks were too much for me at the time as I could get stuck in the middle of the park unable to take another step without feeling like a knife was going in to my groin each time - so I just sat around getting more and more weak and in pain.
By the time my NHS physio referral came round it was only a few weeks from the birth and she just did her best to push my hip back in place (things got so lose it kept flopping back and forth) and it didn't last.
Soon as I could after the birth I started pilates and kept it up until quite recently. That probably makes me sound quite like I'm in to fitness which I'm not though! I just learn a few excercises that help in the classes and then do them here and there in the day, as I didn't have the time/inclination for regular classes really. So for e.g. I'll do 20 squats in the shower, 20 squeezes of my knees with a pillow in between before I go to sleep at bedtime, a buttock strengthening excercise just before I get up in the morning etc, and in the first half of the PG I was doing sit ups whenever I got a spare few minutes.
Get as fit as you can and even if you don't avoid it, I reckon you'll delay it. Soz for big waffle