IMHO the new day packs that put the camelbak inside are worthless because the camelback eats up 50% of your space, and the outside pouch is now streamlined (reducing its capacity as well).
If you are wearing your ruck then put it inside that. Otherwise I've seen some people mount them using Blackhawk Camelbak pouches, but putting it outside the pack. The only disadvantage to this is that it gets exposed to sunlight. If your not using your daypack or ruck you can also use the BH pouch to mount it to your LBV or IBA.
Putting it on outside of your ruck makes it easier to refill the camelback without having to fish it out of the ruck and then put it back in.
There is no perfect solution, its going to be kind of awkward either way. I've tried the combination ruck/camelbacks and they are difficult to refill at night in the field. I would recommend just having a separate camelback outside of your ruck. And make sure you get your name on everything, pretty much everyone has slight variations of the same equipment in the same camo pattern and it can get difficult to find your stuff in a hurry.
When you say "pack" I'm guessing you're using a large back-pack type, or a hiking pack to carry all your food/shelter/water/etc. for a trip (if I'm off here, just ignore the rest of my post).
Either way, you want your heaviest item (which is usually that 3 liters of water) to sit as close to your back, in the center, as possible. If you put it on the outside of your pack you're putting more weight on your shoulders when most of it should be on your waist.
I do the same thing. I strap my camelbak 3L mule system on me first, then I put my rucksack on top of it. You have to make sure the straps are on evenly (I do it using a ruler, measure the pull straps) if you want to avoid it moving all over the place. It may look weird having 2 backpacks, but at least you're hydrated.