Find a few good areas on the map and fortify, keep a strong defensive force at the base. Create a Tobruk scenario where they can't risk moving past those areas and those areas are very hard to take. If you do it right, these positions can also watch the main approaches to your base w/ machine guns and AT weapons.
For the final attack, I find a mixture of transports and heavy infantry works best. The transports keeps them from being suppressed by machine guns, and when they encounter tanks, nothing says scrap metal like 6 squads w/ Panzershreks.
Company of Heroes is a nice game because there is a cover mechanic, meaning they can shell the hell out of your position but if you do things right you're still there in strength. Sadly that works in reverse, you're not hurting a strong enemy position until you have ~2 howizters or ~4 Nebels.
As you may be able to tell, the only RTS I really play is CoH. I also play Hearts of Iron 2/Victoria 2, but those are kind of grander scale. I tend to just try to encircle over-and-over when playing those games so enemy troops are completely destroyed (This really helps when fighting Russia, because you don't want to still be fighting a full-strength-on-paper Red Army when you're stretched all the way to siberia...)