Having a back pack fit correctly is one of the most important facets of the backpacking experience. You can ruin a good trip in a hurry by not having equipment that fits properly.
First of all decide on the quantity and type of equipment you will need to bring. This will assist you in deciding which backpack you need. Deciding on the amount of space needed is also critical, so pretend you are taking a long trip and prepare for space that is conducive to that. Comfort is the most important thing to consider when choosing a backpack. You need to be able to carry the most weight with the least amount of effort.
The backpack that you purchase should be able to distribute the weight efficiently which will allow you to carry the backpack with maximum comfort. Always test the backpack before embarking on a trip to see how well it handles the intended weight. Keep adding weight a little at a time. You are the only one who will know how much weight is comfortable for you.
The straps are another important item. The shoulder straps should carry about 30% of the weight and the hip straps 70%. Make sure the straps are both comfortable and unrestrictive. With the weight...