This deck has two basic ways to win:  build up one or more of the unblockables (Sneaks) to punish the opponent, or build a swarm of tokens oriented toward smashing through the defenses on a turn III Song of Freyalise.

Avoiding mana drought:  mulligan any hand with only one land unless you just can't pass up the other cards and you feel lucky.  With two lands, look to get one or more low casting cost creatures onto the board and follow them up with a Song of Freyalise.  Sneaks will be vulnerable early, so marshal your Spell Pierces, Deeproot Elites, and Merfolk Mistbinders  if possible.

Get out a Deeproot Waters as soon as possible.  Then work the tokens, get out a Song of Freyalise, and go to town on turn III.  +1/+1 counters should generally go to the unblockables unless there's another overriding reason. 

Hopefully you won't get here, but if you do, look for the possible aid of a turn III Song of Freyalise and using appropriate disruption if necessary.

Use the Tricksters for disruption of defenses, and Blink of an Eye to return a Trickster or Pioneer to hand or, if absolutely necessary, to disrupt more blockers.  (Remember, each time you can get to reboard one of them is an additional token from Deeproot Waters and/or a +1/+1 counter from Deeproot Elite, two in the case of the Pioneer...but it's more mana-intensive.)  And with both Deeproot Waters and an Elite out, a single Pioneer is worth three creatures (itself, its token, and a token from Deeproot Waters), and a counter per from the Elite.  The potential for abuse is only limited by your mana. Just don't get overzealous and tap yourself out. As usual, these are just general suggestions; the game state may dictate other strategies.  Sweepers are your mortal (pun intended) enemy...Hour of Devastation, Settle the Wreckage, Goblin Chainwhirler, and their ilk. Either make sure you save a counterspell or Blinks for them or be very judicious about what you attack with...preferably both.  (A Mistbinder will do against a Chainwhirler, though.)

ETA:

Recently, when commenting on this deck on another website, someone asked whether this was an aggro or  a tempo deck.  It wasn't so much the question as how it changed my point of view of the deck, and was borne out in later testing:  it tries to be both, and hamstrings itself by doing so,  So I came up with the Fileted by Fish deck, which is pure aggro.  And now, I'm adjusting this one to go the tempo route.  What that ultimately means is fewer creatures and more disruption.

 Now, what to cut?  Mistbinders, Tricksters, Silvergills, and Elites, are generally integral to what the deck tries to do...and if you've actually played this deck, you'll probably have noticed that River Sneak is a great adjunct here.  With a single Mistbinder out, I've pumped it up to 7/7 (unblockable, of course) before.  That makes it far too valuable to drop...which leaves the Mist-Cloaked Heralds and Kumenas.  I'm conflicted about Kumena, but overall the balance seems to be tipping slightly away from keeping them.  Also, although Curious Obsession is a great card, it creates targets for the opponent and ends up being a lot less effective at times, especially against decks with heavy disruption.  So that cuts 9 cards from the deck.

 As for additions, I'm going to a full playset of Silvergills again, filling out the Pioneers, and doing the same for Song of Freyalise and Deeproot Waters.  That leaves four defense/tempo cards, and for that I'm bringing in the entire playset of Unsummon, and replacing them in the SB with an Essence Scatter, a Naturalize, and two Sorcerous Spyglasses.

 So, for those keeping score:

MD:  -4 Mist-Cloaked Herald, -2 Kumena, -3 Curious Obsession, +2 Silvergill Adept, +1 Jungleborn Pioneer, +1 Song of Freyalise, +1 Deeproot Waters, +4 Unsummon.

SB:  -4 Unsummon, +1 Essence Scatter, +1 Naturalize, +2 Sorcerous Spyglass.

*Updated  for Guilds of Ravnica rotation.  Unfortunately, I can't and won't recommend either this deck or my other UG merfolk deck (Fileted by Fish) until at least Ravnica Allegiance.  GRN took away and gave nothing back.  I don't think it's competitive enough anymore.