A couple of thoughts:
1. I prefer a strap to a chain for comfort and ease of adjustment (a la Jock's suggestion)
2. Wear it high, and consider sliding it to the side for especially rugged terrain as it feels better bouncing on the hip than on the, uh, other parts
3. Consider getting another type sporran, a flattter, stiffer sporran. I almost always wear a hunting sporran like this:



or if extremely casual maybe this:



I found that a Rob Roy holds a lot of stuff, but your stuff will expand to fill whatever space is available if you don't watch it. I prefer a shirt or jacket with pockets for extra stuff unless I am hiking, then it's a backpack or messenger bag (think Indiana Jones!). I have a really nice Rob Roy made of buffalo hide with a Robertson badge (paid over 100 bucks 12 years ago), but I don't wear it anymore. It's just too bulky!