Warcraft III has a decent tactic if you play 1 vs 1 or 2 vs 2 - Build barracks ASAP and start training grunts. Build up a few of them and rush the enemy base with them. Grunts have more health than any of the other basic units, so they will last rather well at the enemy base. Set up a rally point in the enemy base and continue to train grunts to send there. Send in peons with the grunts and start them up building orc guard towers in the enemy base. Orc towers are also pretty strong and well priced. Obviously, for 2 vs 2, you have to co-ordinate this strike with the other person. The best hero for this strategy is the guy who has feral spirit wolves (forgot his name) and he should be coming in with the grunts. Try to not allow their hero to level up by keeping close watch on your guy's health and moving them appropriately. Also bear in mind that you can run back to your towers once they are finished and have some covering fire.
For more GENERAL strategy in RPGs...
-Attacks are only worthwhile if they either capture more resources than the attack itself cost, or have a chance of doing truly devastating damage to the enemy.
-Expanding onto new resources quickly (as fast as possible) will give you the means to overrun enemies.
-Pay close attention to how much units cost and how much damage they do, health, armour etc... Often there are some in game units that are not well balanced and cost rather a lot for what they are.
-Information is king - It's often worth (especially in longer/larger games) sending very cheap units into enemy encampments to investigate possible weaknesses in defences there, how many resources are remaining there and what you might be getting attacked with.
However, there can still be an element of what Computergeek said.
Have you considered trying a game that your opponents haven't been playing for 10 years straight?