I am not that great at DoW, but I have recently been moving up the ranks of the Starcraft 2 ladders.
Basically, the key to victory is to do more with what you have at your disposal.
It's a balancing act.
If you turtle in you have a bunch of static defenses that won't win you the battle.
If you go too aggressive, there is a very real risk of losing your force to their defenses or having a majority of your forces in one location while they decimate another.
The key to victory is to "Start Strong", "Expand Safety", "Upgrade Continuously", "Build Reactively" and finally "Study".
1) Start Strong
If you think about it, every second of the game is a divergent moment, everytime you make a builder instead of a soldier and vice versa you are making a choice that will affect the outcome. The key is to develop a large enough economy to always have a "force in development" if your barracks(or w/e) is idle your doing something wrong, either your neglecting it or your not gathering the amount of resources you should have. (Alternatively, you can find yourself gathering more resources than you need to, if you have "more money than you can spend" your doing it wrong. because that basically means you don't have sufficient defenses to protect an operation of that size
2) Expand Safely
Using rally points, you can have troops automatically arrive in a certain location. Early on you want to assign them to gaurd resources and chokepoints where "rushes" are forced to come in. Once you decide to expand you need to provide security for your expansion, there is no scenario where sacrificing a base due to lack of protection will work in your favor.
3) Upgrade Continously
This is definitely what separates players into skill levels - most players will try to build the most massive force they can to overwhelm and destroy you, very few actually take the time to upgrade the individuals. IF you have 6 soldiers with level 1 weapons fighting 3 soldiers with level 2 weapons, they will probably destroy each other the difference is that your opponents loss in funds is twice that of yours. For every encounter you want your costs to be less than theirs. Always upgrade weapons/armor/shields, it usually costs about as much as a single unit per upgrade so even though it takes time it will be worth it in the end...you can also build multiple upgrade buildings, but if you have that much excess income, then your probably not being forceful enough and you have a lot of "reserve units" just waiting for the inevitable mass strike or God Forbid they make 4 clicks and break their forces into two prongs. Which brings me to my next point.
4) Build Reactively
Scout. Scout, Scout, Scout, Scout, Scout. You should scout. Early on with a builder, later with flying units and stealth units. To consistently win you need to know what your up against, not just have the most devastating force ever conceived and hope that they don't exploit your weaknesses (which are built into the mechanics of the game)
If you know what they are doing, you can "act" toward victory and not just "react" to the Shit Storm at your front door.
So many RTS battles are lost because the player has no idea what the opponent is doing. Something as simple as setting a ground unit on a patrol between you and your opponent's main base gives you an early warning system for minimal cost.
I hope these tips are helpful to you, my final piece of advice is to "study".
5) Study
Go on YouTube and watch matches with your chosen race(especially the ones that have commentary) you will learn very quickly what actually works and what is just "cheese" that will only work if your opponent is trying less than you.
Good luck to you!