I didn't "get it together" until about 2 weeks before ds was born! I was quite superstitious about getting too much too soon (at least, that's my excuse, rather than disorganisation!). However, I was helped by getting most of my stuff as cast-offs from my best friend's (four!) children - and I also had to wait until I went down to Wales to pick it up to know what I still needed.
I got most of the stuff mail order from Kiddicare, which was much cheaper. If it's not on the web site, then just ring or e-mail them and they will literally go around the shop with you on the phone and stock up the "trolley"! The only disadvantage is that you have to be sure that you will want it, as it is expensive to do returns.
Essentials in advance of birth were the car seat, somewhere for the baby to sleep (we were given a crib out of the blue), bedding - but not too much (see comments below re blankets!), some new born clothing for when you are in hospital (you'd be amazed at what you are then given), nappies, muslim squares, cotton wool or wet wipes (I still, with ds now 22 months, just use water and cotton wool followed by some yellow petroleum jelly - I'm still using the original big tub of non-branded stuff that I bought from the chemist - wet wipes are only used when we're out and about) and that was about it!
Essential "extras" were
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a light shawl - my Mum knitted me one and it was great for carrying him, tucking over him in the car seat, swaddling him, laying over hm when he went to sleep in the baby gym etc.
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baby sling
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baby gym - but you might be given one (as we
were)
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bouncy chair (another cast-off from best friend)
One bit of good advice from said best friend was that if people asked, ask for something for when they're about a year old - as by that time the gifts have stopped flowing in! (Also that way, you don't get the problem with the "wrong" seasons that someone mentioned).
I bought loads of fleece blankets as well as the cellular blankets, and ended up giving them away unopened, as initially I just used the cellular blankets and then we put ds into a sleeping bag, so he's never needed blankets. I did get one "pretty" embroidered fleece blanket (more of a shawl) that we still use when out and about as a bit of warmth for him if he needs it.
Didn't bother with a changing table - we've always changed him on the floor (on a changing mat!). I did buy a cheap top and tail bowl though and am still using it.
I've hardly used any sudocream - I've still got some left from the samples you get in the bounty bags, and only ever use it if he's got nappy rash.
The push chair (fancy NZ 3 wheeler) only arrived about 3 weeks after ds was born, but with the sling and a car seat that hadn't been a problem. We also bought the attachment for the push chair that allowed us to use the car seat in it - fantastic!
Althogh the cast-offs included a microwave steriliser, I hardly used it: instead I either used the dishwasher (did buy a wee rack/container for putting bits'n' bobs into the dishwasher) or wet sterilised my pump(I was b/f) - I got into that habit when I was re-hospitalised with an infection when ds was about 2 weeks old and has just started expressing to augment his slow weight gain.
If you're bf, breast pads (washable) and breast shells (for collecting "leakage" ) are useful.
Although I did use the baby bath, I wouldn't go to the expense of buying one. After breaking my back the first couple of times carrying it through the bedroom, I just started using it inside the big bath.
I never bought a proper changing bag - just used a freebie fabric "briefcase" I'd been given at some conference. You would need a travel changing mat though - I was given a posh towelling one from M&S by my Mum, but the more practical one was the wee plastic coated one that my best friend gave me that could roll up very small.
I'd echo the votes in favour of the cordless phone - I had to go out and buy one as ds was such a slow breast feeder and I was always getting interrupted while he was attached.
Make sure you've got plenty of vases - our house looked like a florists for weeks after ds was born!