I haven't painted any, and this isn't a tutorial, just some pointers which might be helpful.
Prime the models white to start with and half the armour is basically done. The white will also give you a good background for the blue, which should look nice and vibrant - try a colour like Enchanted Blue for a good rich blue.
Take your time painting the half-and-half bit - use a small brush (4/0 probably). It might help a lot to mark the line first using a pencil or something - you can always paint it out afterwards if it still shows. The legs and arms aren't going to be a problem, it's just the torso, head and backpack. If you get some blue on the white area, paint it out first with Astronomicon Grey foundation, and then white over the top of that.
For tanks, you could use masking tape to make life a lot easier - applied properly, it will give you a dead straight line. Again, prime white, and then do the blue afterwards.
Hope these help a bit.