Here you will find a list of tips for each destination I have written about, because there is nothing more annoying than turning up at, say, the southern hemisphere having packed summer clothes because, well, it is August, isn’t it?

A hard-learned rule: don’t overpack. I always lay out everything that I might need – which is somehow my whole wardrobe – and then take away the things that I have too many of. Never more than three jeans, two pairs of trousers, two skirts and one dress as basics. Lots of t-shirts, at least two zip-up sweaters and a thin jacket. Half my underwear drawer I do pack – if you are away for two months you don’t want to accidentally run out, but stuff like that you can always buy pretty cheaply, just like socks.
I went to live in Spain for a couple of months, packed fifteen kilos plus ten kilos on flight, returned home and ended up paying seventy euros in extra baggage fees. Top tip: if you are going to be in one place for a long time, pack lightly and buy what you need there – easier to guess how warm or cold it is going to be, and what you are going to really need.
For New Zealand I packed twenty kilos, but as I was mainly backpacking, doing a lot of laundry and not wearing anything fancy, I only wore half of that. I could have saved my back a lot of pain.
But then I went to the UK for a  week last September, having only packed the following: a pair of black jeans and a top for going out, heels, undies and socks, two t-shirts, a pair of trousers and a sweater. Toiletries and a hair drier included, it all fitted in a small suitcase easily carried past the London tube turnstiles. I think I got the knack of packing now.

Barcelona

I have been in summer (one time in July, then August) and winter (February), and both times I wore mainly jeans and skirts!
In summer it can be pretty hot, so take your usual holiday wear: shorts, skirts, t-shirts…
In winter, you can either be lucky and have almost spring weather – when I went in February, it was pretty warm for the time of year: 18 to 22°C (64 to 72°F) – or you can be absolutely drenched with rain.  If you are lucky but quite sun-sensitive, I would pack a bottle of sunscreen, just to be safe.
Either way, don’t go full out on the skirts and spaghetti strap tops in winter, unless you plan to switch day for night – you don’t need to wrap up warm at the clubs, do you? Definitely pack some footwear other than your comfy flip flops, the rain in Spain might stay mainly in the plain, but Catalonia gets its fair share.

New Zealand

I went in July and August, so winter. Keeping in mind that the further south you go in NZ, the colder it gets, you should pack for weather ranging from autumn weather to just plain freezing your nuts off.
Up on the North Island, generally you will get frost in the morning, but temperatures will hover around the 15°C mark (60°F), sometimes even higher, up north (i.e. Whangarei, Paihia and further up). Most of the time you should be expecting temperatures around 10°C, take or leave a couple of degrees.
On the South Island, you will need your gloves even more than on the North Island, as temperatures can go as low as -10°C (15°F). Have a look here for regional temperatures.
Also keep in mind that July is the wettest month, so don’t forget your raincoat, you will really need it!

 What clothing is concerned, these are the things I wore most:
- trainers
- wellies (I was working outside a lot)
- jeans: I took two pairs, lest I be walking around with soaking wet ones.
- decent trousers: three pairs of comfy trousers, suited for warmer and colder days
- a skirt: if it is sunny outside, you might be able to get away with one, girls!
- lots and lots and lots of pairs of socks: if you are going to be hiking, working, walking…you will need   more than you usually pack, unless you don’t mind trudging around with wet feet.
- zip-up sweaters: good for layering; I hate it when I am all wrapped up in a thick woolen sweater only to find out that, damn, it is pretty warm after all, but I can’t take it off because I only have a flimsy t-shirt underneath. Ugh.
- warm winter coat: if it was warm enough outside, I just wore a long-sleeved t-shirt and a zip-up sweater, but the one time I left my winter coat at home, was a big mistake.

Practical tips: New Zealand is pretty windy in places (Wellington for example, so I wouldn’t skimp on the lip chap.
For those worried about insects etc.: not really a mosquito problem, but something much worse: sandflies. Tiny black insects that really give a nasty bite. A tip I picked up: Avon’s Skin So Soft dry oil moisturizing spray tends to keep the buggers at bay. I myself always buy local mosquito sprays, because often I find that the ones I buy at home don’t work as well.
A tip that goes for just about anywhere: take one of those wind-up flashlights; comes in really handy when trying to find your way back to your tent. Mine comes with an extension that allows you to charge your mobile phone – takes a while, but if you are not anywhere near a power outlet or if you have forgotten your power adaptor, it is worth the muscle cramp.
Also, if you are away for quite a while and you have a mobile phone, it would be a good idea to buy a local SIM card. I got a Vodafone one and got fifteen dollars credit for free, which believe it or not, lasted me two months, even though I texted to Europe quite a few times.