Most of this is obvious for us, but I just want to make sure for my own sake.
Build Order:Obviously this is one of the most important aspects of the game especially at the beginning, and for any rushes. I normally do supply pad -> reactor -> barracks (for ODST strat). But I noticed some people build a reactor first. I don't think it really makes a difference but I am not 100% sure.
Troop Movements:
One thing I have trouble with is selecting groups of enemies. I often select all units and attack, but what happens is often my ODSTs get stuck in the back and can't be effective. It's a good idea to use smaller groups and set them up in an arc around what ever you are attacking.
Elephant Idea:
I haven't done this, but even though I can drop ODSTs I still find I have tens of troops trekking across the map. I am thinking if I set up an Elephant just outside their view and keep bringing in a flood of troops. What do you think? Is it worth it?
Explore:
I think the best feeling in the game is when you send out warthogs right away to explore for crates and to see what your opponent is building, and the other guy just has his hog sitting outside. It gives you a pretty damn good idea that they are going to suck. And when you can take their crates it gives you a big leg up.