Some of the ways to get advantage from a list posted months back on reddit. The original thread over there is all the ways to gain any type of advantage/disadvantage from the PHB, below is just the ones giving advantage to attacks.
http://www.reddit.com/r/DnD/comments/29xk0j/5e_summary_of_situations_causing_advantage_or/
-Using the Help action during combat can give an ally advantage in one of his/her own ability checks before the start of your next turn (see "working together"). Alternatively, it can provide advantage on the first of an ally's attack rolls against a monster.
-Attacking an enemy while hidden (if they don't detect you approaching) or otherwise unseen grants you advantage on attack rolls. Conversely, attacking an enemy you can't see has you making them with disadvantage.
-Attacks rolls against targets affected by Faerie Fire are made with advantage if the attacker can see them.
-The next attack roll made against a target hit by Guiding Bolt (before the beginning of the caster's next turn) has advantage.
-The target of Otto's Irresistible Dance has disadvantage on Dexterity saving throws and attack rolls. Other creatures have advantage on attack rolls against him/her.
-Attack rolls against a blinded target have advantage.
-Attack rolls made by an invisible creature have advantage.
-Attack rolls against a paralyzed target have advantage.
-Attack rolls against a petrified target have advantage.
-Attack rolls against a prone target have advantage if the attacker is within 5 feet, or disadvantage otherwise.
-Attack rolls against a restrained target have advantage.
-Attack rolls against a stunned target have advantage.
-Attack rolls against an unconscious target have advantage.
-Inspiration: given by the dungeon master, can be spent to gain advantage on an attack roll, saving throw, or ability check.
-When squeezing through a space one size smaller than itself, a creature has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against it are made at an advantage.
-Any situation the DM says you gain advantage in, so try to be creative and ask.
Combine all of the above with the fact you don't need advantage just an ally adjacent to your target and sneak attack should be pretty near an every round thing.
