all races: if you dont plan to use your hero until you get at least a few army units, it might be worth it to make a barracks(ancient of war or crypt) before your altar, reversing the two in your build order. remember: never let a full health hero go to waste, use it to creep or harass if nothing else. information is a great edge to have, owls, far sight, owl sentries, arcane towers with the magic sentry upgrade, sentry wards, and maybe even shades or a stealthed blademaster can be exceptionally useful. do NOT let anyone whos not on your team expand if you can help it; see previous point.
with humans you want to either get tier 2 or riflemen as fast as possible. there are almost no exceptions to this. when forming a team remember: T1 humans are slightly weak, even if your doing rifles its smart to get tier 2 for the upgrade. once your there it might be smart to kill your footmen off if your foe hasnt gone mass range, as all they are at this point is food, taking up yours and giving it to enemy heroes. footmen are completely useless against ranged units and creeps without defend (which only works on piercing by the way. it does nothing against huntresses breakers or late air except slow you down).
heroes: never ever ever ever ever use a paladin except against undead mountain king is my hero of choice, and is good against rushes or for early creeping. archmage is good and versatile, great ult, but squishy. blood mages are devastating against buildings late game, so against players that tend to cluster them, a blood mage is a good second choice.
with orcs, get a shop ASAP, i mean; by the time your hero comes out if you plan to hero harass. 1 of everything is exceptionally useful in harassing. by the time your first grunt arrives if you plan to creep, scrolls of speed are great creeping accelerators, and that healing salve can be a lifesaver, especially in team games, its practically your rod of necromancy as far as versatility and overall usefulness goes.
BM is the classic harasser, but i find FS can do equally well, and both provide extra muscle and intelligence later in the game. TC is your support choice, with SH taking up second, rarely do i see a tauren chieften as a first hero, but he can work quite well, my experience with SH is limited, but he makes a good second hero. orcs are all about their heroes, dont be afraid to spend a minute on creeping or a few extra gold on relevant items.
with undead: rod of necromancy is a good thing. its versatile and helps with offence, defence, harassing, major battles, and creeping. remember your good regeneration on blight, send injured units back to heal if your DK is OOM or you picked a different hero. nerubian towers are very very good, better than spirit towers for the cost, especially if you plan the towers only to be a delay until your army can get there. even if you mass spirit towers (i cant think of a time when this is a good idea) be sure to mix in nerubian towers for the slow effect.
DH for initial support/muscle, DL for initial support, CL for some rushes, lich is a great second, and especially against a human nuke a dark ranger (mercenary) is not unheard of but undead is not my race. take my advice only if no other is offered.
with night elves: mass archers doesnt usually work very well, a good general rule: never ever ever ever use archers. there are advanced exceptions to this, but i call them advanced for a reason. multiple night elves do not work well on a team, they lack synergy with each other. remember that your the only ones who get siege on T1. a few siege behind a hunt rush can give you an edge going up against an enemy base, or seriously devastate an opposing army of huntresses, since they arent much more expensive and dont take too much longer to produce, its always worthwhile to have at least one in NE mirrors.
DH/KOTG for harassing and extra muscle, POTM/KOTG for support, warden/KOTG/POTM for good seconds