There are those, myself included, who favor shifting some carried weight from rear to front to balance the load. In shifting the weight, you improve your walking balance and the load on both your shoulders and hips. It really does make a difference after five or six hours walking. At least that is what my hips, knees and ankles tell me.
The New Zealand company (AARN.co.nz) makes an outstanding line of technical rucksacks that feature something called "balance packs." These are boxy, 3-liter nylon sacks attached to the front of their proprietary rucksack harness. The packs are large enough to hold 1.5 liter water bottles, a pair of shoes, etc.
The Aussies and Kiwis I observe with these rucksack systems rave about them. I have tried to source them in the US, but they are very expensive and hard to obtain. Perhaps Aarn's distribution system has improved, but I have not seen them more prevalently on my recent Caminos.
I understand that these balance packs may also be available separately for strapping to conventional rucksack harnesses. But, I have not yet seen this done.
BTW, FYI, and apropros of nothing, the folks who own these advanced and very clever rucksack systems tell me that they also call the front balance packs "Pamelas." Evidently, this is a reference to Ms. Pamela Anderson, late of the TV Baywatch series "fame." Hmmm?
Personally, I shift weight by a two-part approach:
1. I carry 4 x .5 liter water bottles on Nite-Ize "Clip & Sip" brackets. These stainless steel / inox clips clamp about the neck of the plastic bottle and allow me to hang the bottles from the front of my harness, at the sternum strap, shoulder straps or on my waist. For reference, one-half liter is one-half kilogram. So, four bottles would be two kg shifted from rear to front.
2. I wear a cuban fiber," Z-Packs," 3-liter capacity "4-in-1 Pack" as a front pack. Originally it rode high like a chest pack, about one-fist below the sternum strap. Then it moved in subsequent Caminos to a "belly pack," over...well...my belly. But, there, it bounced too much...chuckle..
Having finally figured out the optimum way for ME to access and use this very versatile bag, I now hang it in front of my rucksack belt snap buckle. The two straps are affixed to split "O" rings attached to the lower shoulder strap buckle at the end of the waist strap, where it meets the main rucksack. The result looks like a 'sporran' (that purse-like thing that men wear with a kilt). OMG, I hope I got that metaphor right?!
This front-riding sack carries items I KNOW I will use during the day. This includes: my ultralight poncho, fruit, nuts, chocolate, hydration / salt tablets to add to water, guidebook, a sun-hat to switch off with my ball cap, and sometimes my iPhone.
The pack is only available from the Florida manufacturer, a small garage operation. It comes with all the hardware you need to mount, attach and carry this bag in virtually any conceivable manner. The bag is completely set up for maximum flexibility.
It is called the "4-in-1 bag" because it can be attached to the top of your rucksack, carried to the side for touring using the provided longer straps, attached to your harness, or used as a stuff sack. Lord knows these are only the things I have though of. The possibilities are limited by your imagination.
Here is the web site:
http://zpacks.com/accessories/backpack_lid.shtml
The Cuban fiber material is highly water resistant and the quality is outstanding. The pack is also machine washable (cold, gentle, line dry).
I hope this helps.