Rushing - Valid strat, easily defended against though if you know what you're doing. But if you are in RT and your allies don't feel the need to help out it can be hard. I do mostly AT, so that isn't a problem. Even a very well coordinated rush shouldn't be too much to handle unless you tech too soon and leave yourself open. The rushers have the big disadvantage of having to actually attack the base. Each race except UD at the beginning has something in their base to defend against the rush. NE - Uproot AoW, moonwells. Orc - Burrows. Human - militia. UD will of course have their ghouls though so they shouldn't need that much extra help. Trust me, if you know how to play the game then fending off a rush isn't a hard thing to do.
Tower rushing - Scout scout scout, you should be scouting anyways. I got OT'd a couple times, and I haven't had it effectively used against me since. Just scout around your base and if you see any towers or peons you gotta get your allies and your troops over there ASAP. If you're NE be sure to use your ancients, they are great against OT.
Massing - Please. Any diversified force at the hands of a good player will defeat any massed army. In 1v1 especially. In 4v4 AT though, having one person get hunts, another casters, another heavy melee, etc. does work though. That's one case where having one person just mass one type of unit is effective. There really is no way to balance that though, since having one person specialize in one unit and the other in another type will always be more efficient than the two getting both types themselves.
Best way to get good is to play online, tons of strategies and you learn a lot about microing, countering, etc. Single player really doesn't teach you all that much.
There are a lot of cheap tricks, but you'll become better and better at defending them each time and the more you play the less effective they'll be. OT, Massing, etc. are for n00bs. Once you get past that n00b level that's where the strategy is. Microing and macroing. Remember, there is always a counter for everything. That's why scouting is important. You want to know what the enemy is getting, so if they're massing a particular type of unit you can surprise them with the best counter for that.