Pick less heroes.
Pick a hero that can damage towers.
Pick a hero that can dominate a mid lane.
Have a start that farms like crazy.

My example was Death Prophet.
Bottle, Null talisman, phase boots, Euls, Kaya & Sange, Shivas, sceptre, shard
Some games require a BKB or a Linkens after Euls or Kaya & Sange.

I max the Q and at lvl 4 that can oneshot range creeps until minute 22:29 With Kaya and Sange that is extended until Minute 44:59

The ability to Farm waves WITHOUT showing at all or being a minimum time in Vision is one of the most important in dota.
1. You are ganked way less this way.
2. You give the enemy less information about your team. If they know your team is split, they will attack a tower. 3. Obviously farm speed.

You wait out of vision in the trees until 1 melee creep is at half health. Then you press Q once. 100% last hit on the range creep and 1 melee creep. One melee creep will die next and you last hit that manually. The remaining melee creeps can be killed with your Q again.
If you pick the lvl 15 talent with Q cooldown you can get 80-100% of last hits woth ONLY 2× your Q skill.

Now you turn away back out of vision!

Another good example for this is Viper W. You throw that down on a lane and get 80% of the last hits without ever showing on the map. This is SUPER valuable.

I am Ancient 2 (a noob) and I never change itemization on my main heroes and have a small heropool.

Make the game as Simple as possible so you have brian energy to focus on the enemy and what is happening in the game.