I started packing mine at 28 weeks - putting in essentials as I bought them and now pretty much packed (at 31 weeks) just need to tuck in clothes for me last minute, although I like jerin's suggestion and might just do that instead.
These are my first babies, so I can't say whether this will turn out to the best plan or not, but the advice I thought most useful was to have two bags - one with essentials required for first 24 hours or so (stuff for labour and/or making myself comfortable after c-section; toiletries, towel, pads, nightie, dressing gown, slippers etc. plus 1 set clothes for babies) and then another one to leave in the car to be brought to post natal ward with more clothes for babies and more nighties, pads for me, plus nappies, cotton wool, muslins etc. i.e. stuff I will want if I'm in for a few days.
Due to the high likelihood of a catheter with c-section or epidural, you'll need nighties rather than PJs at least at first. I bought some cheap ones with buttons for breastfeeding as I'm more of a PJ girl and unlikely to use them again. Also bought one nice one for cheering up purposes!
Sorry, seem to have inadvertantly written an essay, hope it's helpful and not just a ramble...