I.G.
I feel your pain, I have always had thin hair. Postpartum hair loss is normal, but if it seems excessive check in with your doctor. It could be that your hormones or thyroid are off, which are common post partum problems as well.
For me nothing ever came up medically, but my mom has thinning hair too (and is actually balding at this point), so I know for me it's probably genetics.
As for styling tips: shorter cuts and lighter hair color (if you are white) help. A shorter haircut will weigh down your hair less and make it seem fuller - I mean short short though, not medium length - think pixie cut.
Lighter color will reduce the contrast between your scalp and your hair and thinning areas become less obvious. I always loved to dye my hair black or dark brown...which made it seem even thinner. Now I get highlights instead and it looks much better (hides the gray nicely too, without getting a full color job done).
Good luck.