This game rocks unbelievable amounts.
Get Heavy Plasma and blaster gun and punish those snakemen/mutons with impunity. Also, NEVER sell elerium, and you can gain quick funds my manufacturing/selling medikits. Always have at least one quick deployment group (if you have Avenger, that is) and one reserve group, which can also defend you from unneeded invasions. Get rid of short range radars, as they suck like a 15-year old crackwhore.
Specialize your troops. Majority should have heavy plasma, with one or two blaster dudes (another one can actually carry ammo and have heavy plasma/laser rifle as primary weapon). Always have at least two Small Launchers with full ammo (IIRC the full one man can carry is 12 rounds, I don't know) in case you meet something interesting. Base raids REQUIRE you to have one-two slauncher guys, since only there you can capture Commanders. Laser rifle is a-OK, since it's really accurate and quite lethal.
Lick major countries' ass - China, Germany, France, Britain and USA being to most important.