Buy yourself a backpack that's durable and comfortable, with strong zippers and many compartments. A name-brand bag can last decades, saving money in the long run.
As a general rule of thumb, choose backpacks in the 55L to 75L range if you plan to be away for a year or longer, and something in the 40L to 50L range for shorter trips.
Do your research on the weather of the regions you're going to and the type of activities you want to participate in so you can bring appropriate clothes and shoes.
It's not much fun traveling with a backpack that's heavy as a lead balloon. Try to aim for the 20-pound mark — a very comfortable weight for a day's travel.
Toiletries are often better to buy at your destination than pack in your bag. For the ones you do need, be sure to pack a hanging toiletries bag that can roll up easily.
If you have a skin condition or sensitive skin, consider squeezing in some of your essential items from home, as you might not be able to find the same brands abroad.