I am planning a fastpacking trip this fall. We are planning to power-hike most of a day and partway into darkness (its a long story!)
Anyways, I know which pack I'll be taking and about what gear. I've heard of people using cat litter as a ballast instead of real gear as training. I am tempted to get a 10# bag of kitty litter to carry in my pack just for the extra strength training. An older hiker I work with remembers using books as weight.
One thing to consider is that the distribution of the weight is different with a bag of kitty litter than it would be with your gear, if that makes a difference. Personally, I carry a loaded pack on training hikes.
While it does sit somewhat differently, I don't like to have all of my gear stuffed all the time in my pack for training hikes. But instead of kitty litter, I would recommend gallons of water. That way if for some reason you need to lessen your weight, you can dump the water and still have your container for another day.
Loc: Washington State, King County
Ditto, I prefer water. But what limited 'training' I do rarely involves a pack as heavy as what I'll carry on trail. I'm just sort of trying to get my body used to carrying some weight on my back while walking. Having the exact load, the same pack, having it carry just right --- none of that is a big deal for me, as I'm not all that very serious about training, unless perhaps it's a trip that I know will have a very challenging start for some reason.
The particularly good thing about water IMO is that if your hike is to a local high point, you can lug it all up the hill, then dump out a lot of it to save your in-training knees on the way down without much loss in training value.
The other thing that's popular with any local hiking partner(s) you have is to offer to be their beast of burden while on a training walk. :-)
Loc: Portland, OR
I regularly carry a pack loaded with 'ballast' rather than my actual gear, clothes and food. I use several bulky items that completely fill out the pack to right about 20 lbs. That's about an overnight load for me, which seems right for my training regime, even though I often am out for a week at a time. I don't want to risk an injury from a heavier load because I train by walking faster than I normally hike, but fewer miles.
I would hesitate to use water, because water is much denser than a normal backpack load, which will give it a very different center of gravity than a normal load. Also, a leaky water container could be very unpleasant. I can't say if cat litter would replicate the weight and bulk of a normal load very well, because I don't buy it or own a cat, so I'm not really familiar with its characteristics.
Until I hurt my back in February, I also regularly carried a weighted pack for training.
Since I carry it back and forth on my work commute, I don't want to use a large pack. So I train with an old alpine type climbing pack that still has padded shoulder and hip belts. Since it's relatively small I load it up with dense stuff including a battery and bricks. Last year I often carried 35 pounds which might be even more than what I'd have for a backpacking or overnight climbing trip.
Like Brian, I don't worry about using a different backpack or having the weight distributed differently between training and hiking. I think it still gets muscles and tendons and such conditioned for whatever pack you use while hiking. I do think it's important to get muscles and joints used to carrying weight downhill, so once a week I did a stair workout with the ballasted pack.