First, everyone here is a freak. Second, every freak here can only hypothesize on side stictches. I get them like clockwork during every run, usually within the first couple miles. For me, stitches result from my breathing not being in synch with my cadence which takes a little while for me to do - that's why I get them in the first couple miles. Once I'm warmed up and find my rythym, they don't come back. It could be that your breathing gets out of sync when you start going down hill. Or maybe you tense up a bit on the downhills. Try to belly-breathe - tummy should expand when you inhale and exhale - I find that helps. And make sure you run relaxed. I've even heard that you should exhale when your left foot strikes. Sounds iffy to me but I read that somewhere. belly breathing and relaxing are the things that work for me. And not drinking too much - that'll cause gastro distress and side stitches in me every time. Good luck.

I used to get them more often. I don't know about the issue with downhill running, so I'll just speak in general. I have a trick that consistently works for me to get rid of them. I breathe so that I feel my rib cage expanding across my lower back. I think about this, and the stitch is gone right away. If you try this, let me know whether it works for you. It is more or less the same as belly breathing, but my experience as a woodwind player and teacher tells me that people don't really know how to belly breathe, so often focusing on the ribcage in the back will trigger the right response.

I've even heard that you should exhale when your left foot strikes. Sounds iffy to me but I read that somewhere.

It's not really that iffy, and it always works for me. The theory behind this is that side stitches are primarily caused by the pulling of the liver on the diaphragm. Since the liver is on your right side, you have maximal pull when your left leg is stretching forward. Exhaling pulls your diaphragm up, counteracting the pull, and easing the pressure on the ligaments. (The reverse is true if you have a left-sided side-stitch) I often find when they're really bad that if I stick my right hand into my side to manually support my liver, that works fine as well, though it makes it a little uncomfortable to breathe. But yeah, most of the time, a couple big, deep belly breaths make it go away. I've actually had really awful bilateral ones that, instead of getting better as I ran, always got worse, to the point where it would hurt even to wash dishes, or walk down the street. When I tried to run I ended up doubled over within a mile. My physical therapist claimed these were actually caused by some knotting in my intercostal muscles. I have no idea if that was true or not (I've never heard of anyone else with a similar issue), but showed me how to massage them, and they went away like magic.

I will deposit my 2 cents worth of advice. Take a deep inhale through the nose and exhale hard through the mouth. Left or right foot striking the ground first is your choice.

Also, be aware of what you have eaten before your run. It might be part of the problem. Another, maybe unlikely, remedy is to try some Coke before the run, in case of some gas in your intestines.