On of my favorite ways to deal with an opponents who is running a lot of Sentries is a strategy called Brinkmanship. Briskmanship was a technique first pioneered by Otto Von Bismark as part of his Realpolitik during the period leading up to the First World War.
The goal of Brinkmanship was to push events further down the road to a "Brink" without ever actually reaching it. The belief behind this was that as events moved towards the edge one could confer a very real advantage as people would start to make mistakes.
Such a Machiavellian strategy is directly applicable to a mercantilistic situation like Zerglings vs Sentries. Ultimately he would like to trade his Sentry energy for Zergling deaths at a fairly high rate of return. As a result the more Force Fields you can cause him to waste through Brinkmanship the fewer remaining resources (energy) he has to invest in Zergling deaths (or Roach/Hydra deaths).
The goal here is not to engage your opponent until he has exhausted most of his supply. You would think that it would thus work worst against an opponent who has a high supply of energy, but actually just the opposite is true; the more energy your opponent has the more likely he is to misspend it. An opponent with few sentries will husband his forcefields until a full engagement is taking place. By comparison the enemy with 10+ Sentries will be more willing to throw them away to try and catch a handful of Zerglings.
Ideally you want to engage your opponent, force him to throw down a forcefield, and then retreat with minimal damage. The reason I like to use Zerglings for this, is that many players think of Force Fields as a direct counter to Zerglings and thus are more willing to expend them against advancing Zerglings. Additionally, Zerglings are very fast (a 6 on creep with the speed upgrade) and have the best chance of getting out alive. They are also cheap!
The actual execution of the technique will be to: Run your Zerglings at your opponent, as if to attack, and then dash back just before actually getting to melee range. This feint is more effective if your opponent knows that the rest of your army is close behind. Since Zerglings are the fastest Zerg land unit, it is not uncommon to see them as the harbingers of an attacking force. Ideally, this will convince your opponent that this is an assault and he will expend forcefields. Once he has started running out of forcefields to spend you can begin the real assault.
Remember, don't commit too early, its always better to wait as long as possible to engage as a Zerg because of the speed at which you reinforce.
The Advanced Class
Feinting and drawing out your opponents resources is a great technique, but a better player will start to realize this, so you're going to need a little extra gusto to punch through. There have been a number of innovative strategies to provide that extra "umph;" tunneling Roaches, Doom Drops, Counters at their Main, etc; but one of the easiest consists of splitting your force in half and flanking them. By attacking from multiple sides you're able to achieve much higher surface area and a much larger Arc. The result is that they need many more force fields and much higher micro to deal with the situation. Usually, even a good player won't notice he's fighting a battle on two fronts until one of them has collapsed.