We got a steady stream, something with each pay day. We redecorated the 'nursery' when I was about 16 weeks and then started gradually buying and putting in the drawers/wardrobes.
Bulky stuff is good to get reasonably early because they are necessities and often have lead times, but then you have to think of storage (next2me, pram, baby bath, steriliser, changing bag, bottles/perfect prep if using).
Never too early to get HUNDREDS of muslins! Baby towels, blankets and changing mats (one upstairs and one downstairs I would recommend!) are good to get in as and when but no rush. You will tend to be bought tons of clothes and get hand me downs so they can be later on but nice to have around earlier too :)
This is #2 so we have most things already, but I'm 30 weeks and have been buying bits of clothes and next pay day will be getting stuff for me for hospital (dressing gown, slippers, breast pads, maternity pads, nipple cream etc.) and nappies x