One essential part of this is making sure that the Squigs will strike ahead of, or at least at the same time as, the enemy. Anything with an orclike initiative of 2 will make for decent targets for your Squigs, as will units that are sporting great weapons, which are all too popular these days. In fact, some of the most cost effective and feared units in Warhammer as of this writing: Chaos Marauders of Khorne and Dwarf Warriors, equipped with Great Weapons, will both lose to a Squig Herd if the model count is about the same, and the Squig Herd will actually cost you less to field. Squigs can even do a fine job against Grave Guards of all types.
On a similar note, enemies like Witch Elves, Swordmasters or Black Guards will make a mess of your Squigs, as might other, less Elite units that will strike ahead of you. If you do find yourself going up against such foes, a spell such as Itching can really save your day. For Squigs, striking ahead of your opponent is key to victory.
Sadly, Squigs tend to get shot up a lot and will not be able to handle artillery fire like Orc units will. True, they are immune to psychology, but a couple of stone thrower shots will lessen your effectiveness greatly, or even worse kill off your Herders and make your Squigs run wild amongst your own troops.
However, as a Greenskin commander you will have to get used to your troops having weaknesses, and Squig Herds (contrary to Orcs or Trolls) will survive spells a lot better. A Purple Sun or a Pit of Shades won't hurt as much, and Squigs make for terrible targets for a Dwellers Below spell. Also, since you won't be able to field characters in your Squig mob, it will rarely be a game breaking loss if you do happen to lose your Squigs. In most cases, the Squigs will get their points back even if it does kill them in the process.
Unit size: There are a few ways that you can field a Squig Herd in hopes of getting something useful out of them. In a horde army with plenty of anvil mobs for your enemies to get stuck in against, smaller units of Squig Herds can turn a battle in your favour by hitting the flanks of your enemy. For this role, I suggest running units of 14-21 Squigs with 7 Herders, deployed 7 wide. 21 S5 attacks for somewhere between 120-150 points is a bargain no matter which way you look at it. Smaller units will also make for less tempting targets for enemy artillery and spells, which is one way of keeping them safe yet effective.
If you're feeling a bit more bold however, Squig Herds are great in Horde formations. 30 Squigs with 10 Herders deployed 10 wide will come in at less points than 8 Trolls or 21 Black Orcs (the special choices they will likely be competing against for the army spot) and will tear through enemy units that you might otherwise have trouble against. The afore mentioned Horde Marauders or Dwarf Warriors will not be able to stand up against your Squigs, and only very artillery heavy armies will be able to take out the Squigs before your army reaches theirs.
In previous editions of the army book, Squig Herd were also quite popular as "Squig Bombs" that only wanted to march up close to the enemy and Go Wild, inflicting D6 S5 hits on units in the near vicinity, but with the 10+ model requirement of the new army book this is isn't longer a good option in my opinion. If you're looking for Squig Bombs, take a Mangler Squig instead, and leave the Squig Herd for eating faces in combat.