Camping

Room-camping.

Rush the hostages and bring them to the room as fast as possible. Generally speaking, injure the hostages, so errant grenades or bullet-spam will kill them. The CTs may or may not stop spamming in future rounds because of lost money. Only 1 hostage is required to block a swinging door, so this one should usually be left at full health. If more than 1 hostage is blocking a door, then there is a lower risk of the hostage opening the door and running out.
Camp behind objects, so bullets do not cause full damage. Don't forget that a room can also be spammed from above or below.
Muzzles usually appear through the door before the CT opens it, so Ts always have the advantage like that. If the CT survives and sees the camping positions, consider moving as the CTs will spam.
Most of all, be patient. The CTs must rescue at least 1 hostage to win the round. If you have all the hostages in the room, then you have nothing to lose.
In following rounds, alternate camping rooms. For example in cs_militia, change up between the bathroom, garage attic and rushing.

Area-camping.

The best examples of area-camping are in de_dust and cs_asssult. In Dust, Ts should protect the hallway and underpass as CTs are generally too impatient to wait the entire round (even though they could easily win by camping the bomb sites).
In Assault, Ts can spam grenades as long as they want (for 90 seconds, at least). And since CTs must move forward, the grenades are usually not wasted, especially at the vent and doors.
Most importantly, be patient, because it's unlikely the other team will be. Then, pick them off as they appear. Any map can be area-camped like this, even if it's the CTs on an assassination map.

The bomb.

Generally, always plant the bomb on top of boxes that are only accessable from boosts. If that is not possible, then plant the bomb where you have a clear line-of-sight from a nearby camping spot (like de_nuke or de_train).

Doors.

Open blocked doors with your back and it will open as if opened from the other side. If the door is blocked by hostages, then press 'use' and the hostages will move with you. Depending on the type, doors can be opened by discarded weapons and thrown grenades.

Ladders.

Crouch while climbing ladders as it reduces noise by more than 50% (estimating by ear) but does not slow you down.
Don't be afraid to jump onto ladders. No matter how far your drop is, if you catch a ladder before you hit a solid object (like the ground or your teammate's head), you won't take any damage.
To move faster on a ladder, do the following - point your crosshairs in the direction you are climbing, turn so you are sideways to the ladder. Then, in addition to pressing 'forward', press the 'strafe' key that is opposite to the ladder.

CAMPING

BACK

Weapons of the camper

When holed up in a room, think about buying weapons which penetrate layers. Also consider buying the Desert Eagle, so your sidearm can also be used to shoot through objects. Consider buying more than 1 primary weapon if camping in a single room. Don't forget to buy ammo for it.
Automatic weapons are a must, especially when there is a buy zone within the camping room. Spam bullets everywhere and then buy more. For this reason, the Para is a great defensive weapon.
Bolt-action sniper rifles are generally useless.
While room-camping, grenades are not as useful as area camping, where you can spam grenades. There is also the chance of the grenade killing hostages, especially if they've been weakened. But, a well-placed grenade can be useful, room-camping or not.
Scoped weapons are essential in area-camping. For example, camping the last sewer entrance near the T spawn, use the Commando or AUG to snipe CTs who exit from the sewers near the garage - they usually are too late before they realise your position. If an enemy pops up directly in front of you, you won't be as disadvantaged, especially if there are more than one and/or they have automatic weapons.

Weapon penetration

1 layer means that the bullet will penetrate 1 layer and embed in a second, causing damage.
2 layers means that the bullet will penetrate 2 layers and embed in the third, causing damage.
No weapons penetrate more than 3 layers.
Bullet damage decreases with each layer. Layer thickness has no effect on damage - it is a fixed value.
No matter what it looks like, submachine guns cannot shoot through doors, even though sparks and bullet decals appear on the other side.
See CS Unclassified (see links) for more detailed information on weapons characteristics.

Pistols - Desert Eagle, 1 layer
Shotguns - none.
SMGs - none.
Asssault Rifles - All, 1 layer.
Sniper Rifles - All, except 550, 2 layers. 550, 1 layer.
PARA - 1 layer.
Heavy Explosive grenades - Anything within blast radius, regardless.
Smoke grenades - Anything within range, regardless of objects.
Flashbangs - none. Frequently, less than none.

Shared ammo

Weapons share ammo. Generally, you'll simply switch weapons if you run out of ammo, but if you have a weapon you don't want to lose, then look for a weapon that shares its ammo and pick it up. Then, pick up your previous weapon and you'll have picked up the extra ammo.
Secondary and primary weapons also share ammo, so it is possible to have a USP with more than 100 spare rounds, etc.