Step one is be smart about picking him. You don't want to fuck around with AM when the other team has super hard counters like Bloodseeker or Riki. Anything where you know they're going to wreck you, and you specifically, before like 25 minutes. I find that even supposed counters like PA, Void, or Ursa you can kind of fight around and be smart about.

It's kind of hard to quantify what is and isn't a good AM game based on picks, but generally I look for this:

1. Nobody on your team has either picked or called safe lane farm
2. No existing strife (two people fighting over mid, someone calling GG because of bad picks or annoying heroes on other team)
3. Other team doesn't have any heroes that REALLY SUCK to play AM against: Bloodseeker, Night Stalker, Riki, Troll Warlord
4. Other team doesn't have more than one hero that it kinda sucks to play AM against: Axe, Bristleback, Bounty Hunter, Huskar, Lone Druid, Outworld Devourer, Phantom Assassin, Razor, Sven, Spirit Breaker, Slardar, Viper, Ursa
5. Your team doesn't have an obviously terrible lineup

If you've done those 4 things right, you should probably be okay to pick AM. It also rules out most games. Obviously you can't always wait to pick last but you can get a sense from the first few picks as to whether your team is going to be okay (decent mid, a support or two, nobody trying to jungle Zeus or anything), and whether the other team is going to be really scary to play against (tanky heroes, long silences, low mana pools, minus armor, high physical damage).

You can be less of a tightass about those requirements obviously; sometimes somebody randoms Lone Druid or Meepo and they can't micro to save their life so they just feed all game, but you don't know that at the pick screen and I'm just telling you the safest way to play it. I hate losing with AM, I don't know why--probably because of muh precious 70% winrate--and I'd rather lose with other heroes, so I'm hesitant to pick him.

That's probably the most important part, is picking your battles, but there are other things you can do in the early and mid game to help as well.

1. Keep your teammates from intentionally feeding until it doesn't matter anymore. You might lose the laning stage, but it's okay as long as nobody gives up completely. Don't rage at anyone, try to keep everyone calm.
2. Buy a Quelling Blade at the side shop. Do this as soon as you can. If things go anywhere near planned, this item is your best friend and you will keep it in your inventory almost the whole game, and will replace it to become six-slotted (minus BoTs).
3. Finish Power Treads at the side shop early. Brown boots + Battle Fury Antimage sucks, shit like that is one of the reasons people say AM can't fight early, because he's walking around with no HP or move speed. That said, keep them on Agi and learn to tread-switch to Blink and for fighting.
4. Buy a Vlads. Just do it. Get it before Yasha. This is more my personal preference but it's so good. It keeps you at full health while farming, makes you push harder, you lifesteal off of Mana Break damage, whatever. It's awesome, seriously.
5. Heart sucks, by which I mean it's extremely situational. I used to build it against, like, Faceless Void back when Chrono disabled evasion but now I'd probably rather have Butterfly until he picks up MKB. People who go Battle Fury --> Treads --> Manta --> BKB --> Heart all with no quelling blade make me want to vomit.
6. USE Quelling Blade. Like, use the active to make your farming more efficient. There's one tree between the dire Large and medium camps that you can cut down to walk through so that saves a blink, and there are a couple trees you can cut down near dire safe lane so you can kill the large camp between creep waves. Similarly, cutting the trees to the north of the Radiant small camp makes transitioning between the small and medium camps faster.

Or you could not listen to me and just play the hero a bunch and learn what works and what doesn't. That's probably better than just trying to imitate some random guy on the internet. Download some replays from Black^, Illidan, or Beesa and watch their player perspective.

Source: Bunch of games at a little under 4k MMR, was ranked 250th on Dotabuff.

http://www.dotabuff.com/players/122350285