Make sure you have a good lineout. Especially short variations. If you are up against monsters, a 4 man lineout with a bit of movement can often tip the scales in your balance. I think a good lineout can be more important than a dominant scrum. I would be happy with an adequate scrum and a dominant lineout.
I love the short lineout, it gives you options for a mid field crash, or not, which keeps the opposition guessing.
I am also a firm believer in 2 pods of lifters and jumpers in the u16 and below. It is very rare to find a hooker that can drop the ball on to a jumper standing at 4. Instead it goes flat. A good 2 jumper may well get it.
Always compete on their throw in. Referees often let a not straight throw in go if the opposition does not compete.
Make sure you have guys who can lift at the front and the back. They can be funny about the front, and often the back lifter can get kicked in the buts by a newbie jumper flailing legs.
I spend ten minutes, at least, every single practise, running through lineout calls and moves. I often have nights where the reserves are in key positions and everyone is playing their back up position. As mentioned in other posts, every member of my teams must be able to play two positions. Forwards must be able to lift front and back, or lift and jump. I took a team to the South Island U15 tournament this year, on the way to the finals the boys worked out with the reserves still able to play (after 3 games in 3 days against some of the top teams), there were something like 20 different combinations of lifters and jumpers I could field.
It means too that in case of injury, if a player does not think he can make a crucial jump, there should always be enough players, at least one, on the field able to step in to a key spot. If I am dealing with an injury, the reserve can just run in and join the correct pod, maybe with a slight positional shuffle.
Have a lineout call for a quick throw to 2. Good for when opposition are shagging around and not organised.
