Derailleur problems - rear mech slipping forward
I have started to notice a problem in my rear mech - the link arm screw that holds the mech to the dropout seems to slip and cause the whole derailleur to swing forward until the cogs are sandwiching the chain against the sprocket. Now I'm no doctor, but I can easily say that sort of cramping is bad. 


Strange that this would be where things are coming loose. My recommendation is to remove the rear wheel and chain and make sure nothing else is coming loose either. If it's just the connection to the hanger, check the threads on both the derailleur and the hanger as one or the other may have damage or contaminants. Clean and lube/locktite the bolt and re-install the derailleur. If you continue to have problems, I'd have a mechanic check it out.


  Secondly, how tight must this screw be? Wrenching it as hard as I sensibly and safely can with a park tool only holds for a while, and then the problem repeats. Also, how would you measure tightness? (I feel like there is an obvious answer there that I should know already...)


Park Tool recommends 8-10 Nm or 70-86 lb-in. You can easily measure this with a torque wrench, you can get a cheap one from a hardware store or go for a more expensive one if you want something that lasts.


  Finally, given the problem occurs on the trail, and specifically I think on landings and rough patches, is there something in my riding style that could help prevent damage. Advice is welcome, but I'm not an absolute beginner. 


Just trying to stay smooth is about the only advice you need. And don't land to flat! That puts a lot of stress on everything.