Keeping your pack to 10kg or less...
Pack three tops, three bottoms, a jacket and a light wool top - long pants that zip off into shorts or light-weight pants and shirts that will stay rolled up help maximise your options.
disposable plastic raincoat if you are heading to the tropics -
they may not be a fashion statement but they are much easier to
carry than an umbrella and take up much less space than a normal
Pack a sarong - you can wear it, use it as a curtain, transport your washing to the laundry in it, use it as a portable changing tent in shared accommodation... you get the drill
Stick to two pairs of shoes - good quality sneakers and a pair of sandals, slides or jandals that are decent enough to wear out on the town.. and if you think you are close to the airline's baggage restriction, wear your heaviest shoes and clothes on the plane
Use small plastic bottles (around $1 at Payless Plastics) or old pill bottles to decant toiletries into - toothpaste included!
Ditch extra packaging - only take the medication you need but leave important prescription medication in the original bottle. Take general tablets (Nurofen, Panadol etc) in a zip lock bag (which will pack flat) to take the number of tablets you need... plus a few spares for emergencies - remember to write the name of what it is and the dosage on the bag so you remember!
Don't take your big bottle of perfume - buy a refillable purse spray (the Body Shop do a small silver one for about $9) and put your favourite scent into it. You'll save space plus you'll avoid having your glass perfume bottle smash in your luggage
Take biodegradable soap so you can wash your own clothes - that way you can pack fewer socks and undies
Only take one book and exchange it on the road - most backpacker hostels and travelers cafes will run a book exchange.