I like to have my back pack near me so I can reach it from the hammock without getting up.
If you use a back pack cover (not certain on the english word for it) you can put it on backwards so all the pockets is easily accessible and covered from the ground moist.
I try to have it standing up and I to that with to sticks crossing each other (like a tripod with the back pack as the third leg). I you use walking sticks you can just as easily use that if your not using it for your hammock (for example the bridge hammock).
Since I'm getting (if all goes well) a dog in the summer I bout a primus shelter for the dog and will also use it for the backpack to keep it dry.
But the back pack cover with to sticks works wounderful for me and makes it easy accessible. I use this technique even if I'm not using the hammock. My backpack have a built in cover and I have an extra with me so I can fully cover the back pack back and front if I can't have under a tarp.