Shiva:  When my internet sucks and I want to experiment with new builds, I use the Skirmish AI Mod to practice against Hard/Nightmare AI.

Its not the same but it comes as close as i've figured out whether a strategy or build might be viable against real people.  You never know though... I played some multiplayer yesterday and some of the builds I had come up with were rubbish even though they dominated the AI better than viable multiplayer styles.

If you want to try my practice method follow these steps:

-Download and install skirmish AI mod... its in the modding section of this forum.

-Set up a 3v3 on Cataract with AI set on low gold/very high HP.

-Go against the best: Unclean Beast, Erebus, Oak... give yourself Sedna and Unclean Beast.

-Your AI allies should be set to easy, your opponents set to hard... I put Oak on nightmare for an extra challenge.  You might want to start out with normal AI on your side until you get the hang of it.

-Avoid cheesy AI abusing strategies and try and play against them like you would against real people and it might make for a decent introduction to the game.  You'll learn a few things that will definitely translate to multiplayer such as: Its easy to win with good Demigods like Beast, impossible with crappy demigods like Queen... 2v1 with you as the 1 is very bad! Which will also help you learn that dying is the easiest way to lose this game for you and your allies.

There are some things that won't translate to multiplayer at all so be careful.  AI isn't as aggressive as real players so realize that in multiplayer people will kamikaze kill you because if they kill you and then die to the tower you're hiding behind right after, they receive gold and experience while you get nothing cause you're dead.  Also against AI getting priest upgrade very early equals win!  In multiplayer it will lose your team the game.  Just watch out for that sort of thing.