When and what to push

In the early game, Zealots are the slowest units,
but make excellent meat shields to Marauder fire.

Immortals are great to use for early pushes in all
match ups. They do 50 damage to Roaches,
Stalkers, and Marauders

Pushing is something I have been having trouble with recently. I am still in the mentality that if I want to win, I should be pushing really early on, something that carried over from all those matches I four gated every time. In fact I rarely had a match that lasted longer than 18 minutes. I would always push when I felt I could do enough damage. What I didn't take into account was things I should wait for to make my push 100% better.

What to wait for
Things like waiting on a Forge upgrade, or a couple of Immortals, or even Charge. Things like that can make that push that much more effective. So now instead of pushing with a large force and surprising your opponent, you know have an upgrade that makes that large force much more powerful, and that upgrade isn't even noticed until after the battle starts. You might see 6 Zealots assembling at your front and think oh all I need is 3-4 roaches and I can kite em around, but what you don't see is 6 Charged Zealots assembling at your front and suddenly kiting is suddenly near impossible to achieve.

You never want to push into your opponents base without scouting what he has or what he was going for. Constant scouting can help you determine what you should be making or if that push you were making was going to work.

Also, don't plan to defeat your opponent early unless you know for 100% you can. In most cases its more beneficial to wait on that upgrade and do small pushes to help you establish that expansion. However waiting to push until you have a higher tech unit is generally Ok. Good units to wait for would be the Immortal, Collossus, Hydralisk, Siege Tanks, Medivacs.

So far we have  two elements that should be considered before pushing, upgrades, and scouting. Going for specific unit counts before you push is only useful if your going for a build where all of your resources are going straight into unit production, like the 4 warp gate build because your objective with pushing is gaining an advantage, not losing it.

Defenders advantage and the proxy Pylon
Another problem with trying to push early on to win the game is something called the defenders advantage. No matter how many units you have, your opponent will be able to reinforce himself faster if he is defending against your push. This is why proxy Pylons are so GOOD! I tend to always build a Pylon near the Watch towers. They are usually the half way marker between our bases and cut my reinforcing time down by half.