Rock-types are some of my favorite Pokémon (especially the Fossil ones), and they can be VERY powerful.

Rock-types are actually pretty diverse, with only some being those slow, sturdy Pokémon with high Defense (like Golem and Rhyperior), and many others that aren't like that.

1. The Sandstorm Strategy

Generally speaking, Rock-type Pokémon do have high Defense, but their Sp. Defense isn't usually as good.

In a Sandstorm, however, their Sp. Defense increases by 1.5×, which can be VERY useful for making your sturdiest Rock-types even sturdier!

The best way of causing a Sandstorm is with the Sand Stream Ability (especially in Gens 4 & 5, where the Sandstorm it creates lasts forever), and the strongest Pokémon with this Ability is Tyranitar.

Tyranitar has high Attack, high Defense, and even high Sp. Defense, so combined with it's Sand Stream Ability, it's arguably the most powerful Rock-type Pokémon of all!

While there aren't many Rock-types that get Moves or Abilities that benefit from Sandstorms, you can always throw in some Ground-types (many of which do get such Abilities) and/or Steel-types, just like Steven Stone does.
(Even though he's more of a Steel-type Trainer than a Rock-type Trainer, I suppose.)

2. Offense

Rock is actually one of the best offensive Types there is, and it's Super Effective against Fire, Ice, Flying, and Bug-types, while you only have to worry about Fighting, Ground, and Steel-types.

For faster Rock-types, Rock Slide is a great Move (especially in Double or Triple Battles), due to the 30% Flinching chance, but Stone Edge is better if more damage is preferred.
(Or if your Pokémon is too slow to make the opponents Flinch anyway.)

For those that can learn it, Head Smash is an AWESOME Move, particularly for those Pokémon that have the Ability "Rock Head".
(Like Tyrantrum, Aggron, and Relicanth.)

Rock Blast is a slightly more accurate (albeit weaker) alternative to Stone Edge, with the added benefit of hitting the opponent multiple times (which can be handy sometimes), and while Rock Tomb and Smack Down aren't that strong, their secondary effects can sometimes be handy.

For those few Rock-types with higher Sp. Attack, you don't have many choices of Moves (Rock-type Moves, anyway), and it's basically just a choice between Ancient Power (my preference, due to that 10% chance of a Stats increase), or the slightly stronger Move, Power Gem.

As far as covering your weaknesses goes, Ground-type Moves like Earthquake or Earth Power are usually good options for dealing with Steel-types, while Fighting and Ground-type opponents can usually just be hit with powerful Moves that take advantage of your Pokémon's secondary Type.
(Especially if that Type happens to be Super Effective on them.)

3. Defense

In addition to the Sandstorm Strategy, and (typically) having a high Defense Stat, Rock-types also often have useful defensive Abilities, too.

Sturdy is an EXTREMELY useful Ability (in Gen 5 and beyond), and it's basically like having a built-in Focus Sash.

Solid Rock is another great Ability, which can be combined with damage reducing Berries to greatly reduce the damage taken by Super Effective Moves.

For example, Rhyperior takes 4× damage from Water or Grass-type Moves, but if it has Solid Rock, and is holding a Passho or Rindo Berry, the damage is only 1.5×, 62.5% less than normal.

As always, your Pokémon's secondary Types can make a big difference (both positively and negatively), and this is certainly the case for Rock-types.

While Rock-types may have good defensive Stats and Abilities, they have many weaknesses (Water, Grass, Fighting, Ground, and Steel), and it's not uncommon for these weaknesses to overlap with those of their secondary Types, which can be a MAJOR problem.

Tyranitar, Aggron, and Diancie are just a few examples of really powerful Rock-types that have 1 or more Types of Moves that inflict 4× damage against them, and while the techniques mentioned above can mitigate that somewhat, you'll still need to have a wide variety of secondary Types, to make up for these Pokémon's weak points.

Rock/Flying-types, like Aerodactyl and Archeops (both of which are VERY fast and powerful, and would make excellent additions to any Rock-type Team) aren't weak against Grass, Fighting, or Ground-type Moves, for example, while Rock/Water-types (like many of the other Fossil Pokémon) are more resistant to Water and Steel-type Moves than other Rock-types.
(They take 4× damage from Grass-type Moves, though.)

4. Stealth Rock

IK I've been somewhat dismissive of it before, but it is a great Move.

It damages your opponents Pokémon as they are sent out, and it normally inflicts 12.5% of their max HP in damage.

Additionally, it's affected by Type Matchups, so if it hits a Pokémon that takes 4× damage from Rock-type Moves (like Ho-oh, or Volcarona), they lose 50% of their HP right when they're first sent out!