Tips on Winning Pub Games

Play a Carry:  especially in lower leagues, few people can last-hit decently, and fewer still can close out a game quickly.  Chances are, you’ll eventually get levels and farm, and a carry hero will serve you very well if the game goes long.  Try to pick carries that are useful in the midgame as well (stay away from PL, Antimage, Morph, etc unless you are very familiar with one of those heroes).  The later the game goes, the better it is to have more carries on your team (even if they are just semi-carries like Tusk or Axe, for example).

Play a Healer:  heroes like Abaddon, Omni, Legion, Necrophos, and Witch Doctor are valuable additions to pub teams, where the opposition usually has trouble coordinating their burst damage.  Healing can really turn a gank or teamfight around, or allow you to sustain a push.  Simply buying a Mek can also help tremendously.  Some pro teams go Mek first with their mid (Shadowfiend or Viper for example), because the item is so good in those first teamfights.

Play a Teamfight Hero:  chances are that there will be teamfights that decide the game, and some heroes have really good teamfight abilities.  Taking towers after a teamfight win, before you opponents can do the same, can allow your team to snowball (see Heroes with Short Ult Cooldowns).

Play a Jungler:  if no one has taken a jungler, consider doing so.  It’s very unlikely for a pub team to really shut down a jungler hard, and the mere fact of having a jungler means that more XP is available to your team, because the top and bottom (dual) lanes aren’t splitting XP.  However, you have to be sure that at least one player is willing to solo lane against a duo, and picks a hero who won’t get destroyed by doing so (see Strong Offlane Heroes).  The trick with a Jungler is to LEAVE THE JUNGLE occasionally and GANK A LANE.  Adding a jungler weakens your lanes, and you have to make up for that by helping the lanes as soon as you can (usually by level 3 or 4).  If you're going to gank mid, consider being a bit more next-level and use a Smoke.

Learn a Mobile Hero:  for some reason, heroes like Nature’s Prophet, Tinker, or Bloodseeker, who are highly mobile, tend to win games if played well.  These are SINGLE heroes that can turn the game around for their team, but the trick is to know when to split-push and when to join or start fights.  You need to develop good location and map awareness to play these heroes well.  If that’s not your strong suit, then play a healer instead, and follow a carry around.

Stay with your Team:  if you’ve noticed that your team has grouped up, don’t leave and go off on your own.  You’ll either get picked off, or your team will lose the next teamfight 4v5, and it’ll be your fault.  Even if everyone is just milling around for a bit, be patient and don’t just wander off without saying anything.

Don’t stay too long:  This tip can only be acted upon once you have some game awareness – there will be times that you’ll just throw your life away if you try to take one more objective, or kill one more fleeing hero.  Take the easy objective, and then go back to regen or farm; only when your team is undamaged, with ults available, should you keep on pushing.

Buy a Ward once in a while.  Wards win games, but it’s rare in pubs that one player will volunteer to buy them all game.  You can help your team win by buying one, for 75 gold, once in awhile (it’s 2 creep last-hits…)

Buy a Smoke and Gank with a Buddy:  Some pub teams actually DO use wards, but if you and a buddy buy a Smoke and go ganking, you’re fairly likely to get a kill.

Match their Jungler or Ganker:  if the other team jungles, your team should jungle as well, or dedicate someone to gank their jungler repeatedly.  It does less good to kill several heroes once, than to completely shut down one hero who could otherwise become a big problem (like Nature’s Prophet or Ursa).

Buy a Shadow Blade:  I don't know what it is about pubs, but no one wants to buy Dust or a Gem.  Thus, the Shadow Blade is among the best escape and ganking items that exists.  Just don't forget to attack once to get the bonus damage, before using any nukes.

Do not Blame Your Team For Losses:  you can always improve your own play, even if it’s just increasing your last hits while in lane.  While other players may make big mistakes, or be far less skilled than you think you are, you are being matched with them because your team should have an approximate 50% chance of winning (or at least 40-60%), given the matchmaking algorithms in Dota.

Pick something to practice every match:  It's a good idea to have a personal goal every game, whether it's to improve your last hitting, improve your lane harass, or to reduce your death count.  Good players who are get better quickly, are the ones with specific goals.