John Mosby brought up some good points about getting close to an enemy force negates some of their IDF capabilities. This is true, it is a tactic that the PLA used quite effectively in Korea and was used by the NVA/VC to negate American IDF in Vietnam. This isn't a tactic that the insurgents in Afghanistan or Iraq have used very much, although it remains a valid technique, John also pointed out that even with dismal marksmanship training most Americans are good 200 meters and under which may explain why the Iraq/Afghan insurgents don't use this technique.
Indirect fire is the "Big Hammer" on the battlefield. Indirect fire comes from mortars, cannon artillery, rocket artillery, close combat aviation (helicopters), and close air support (fixed wing).
Mortars. Mortars are a "high angle of attack" platform controlled by the ground maneuver commander (Platoon level in the USMC, Company level in the Army). High angle of attack means the tube is angled between 45 and 90 degrees. Mortars are the quickest IDF to respond to enemy contact. The effects of mortars depends on the size of the round, a 60mm mortar is barely better than a hand grenade, an 81/82mm mortar is about 50% again as effective, and a 120mm mortar is about twice as effective with a ground impact fuze. With an airburst proximity fuze they are a bit more effective. Time of flight can be anywhere from a few seconds up towards a minute (or longer for some heavy mortars).
Cannon artillery. 105mm, 152mm (Soviet) and 155mm artillery can come from either towed howitzers or self propelled howitzers. Where you are in the world kinda dictates what you can bring. The deserts of Iraq were just fine for self propelled Paladins, but the mountains of Afghanistan kinda dictate air mobile M777s. Cannon Artillery can fire high angle or low angle attacks. Within a certain range envelope artillery can fire a "high/low" mission which will deliver one round high angle and one round low angle of attack. German self propelled artillery with an automated fire control system can deliver up to 5 round on target at the same time in this manner. Cannon rounds are usually 3 to 5 times more effective (lethal) than 120mm mortar rounds, able to pop road wheels off of tanks. GPS guided "Excalibur" rounds are accurate within a few meters. Time of flight can be well over a minute.
Rocket artillery. This is the long range, big payload, expensive to shoot hammer. From 80 kilometers away 50 pounds of high explosive can come through a roof or a window with single digit meter precision. Great against fixed targets, not so great against those not in a fixed position. Time of flight can be measured in minutes.
Close Combat Aviation. Any armed helicopter can serve as a weapons platform in the sky. The Soviets have a great system in the HinD, but the Apache carries a bigger main gun which can more easily defeat armored ground vehicles. Either way, CCA is much more responsive against mobile targets because there is a pilot in contact with the ground commander (or his FSO or RTO) who can make real time adjustments based on situational understanding. Getting CCA into the area can take some time, but once in the area they like to stay and play until they run out of things to shoot at the targets on the ground. Multiple aircraft can service a target continually, and you calculate this by the distance to target, time to refuel/rearm, and how long a load of ammo will last at the target. CCA is bad news for insurgents with no air defense capabilities, hence insurgents using RPG-7's and sometimes getting lucky.
Close Air Support. This can range from something low and slow like an A-10 or prop driven fighter, to a sleek and fast F16. This is where 500 pound bombs come from, and it is usually very effective on target. There are different types of "terminal control" for the attack. Suffice to say CAS brings a lot of firepower, but they can't stay and play as long as CCA, and require more coordination to shift fires based on input from the observer on the ground.
So, with this "Big Hammer" in the toolbox, how do insurgents in Iraq, Afghanistan, Chechnya manage to stay in the fight? Planning, preparation, and rehearsals.
Notice that it takes time to get the Big Hammer into the fight. If you attack and then leave, the conventional force may not have time to get rounds on target. This means to be effective you need to keep your operations within a disciplined time window (the longer a fight goes on the less likely it is going to be successful for an insurgent force). This means you need to plan your operations with an eye for avenues of approach and egress. This means you need to prepare fighting positions, in Grozny they barricaded the ground level entrances to buildings and sandbagged the roofs on the third floor, then connected buildings with tunnels. This kept the Russian ground troops out of the ground floor, provided some protection against IDF from above (high angle of attack inside cities is the general rule) and allow Chechen rebels to maneuver between buildings with impunity.
So, avoid detection, harden your fighting position (learn from the Chechens and don't fight on rooftops), avoid being pinned down. So to sum up, hide with pride, conduct operations with speed and precision, don't get pinned into a fixed position where the Big Hammer can pound you flat like a recalcitrant nail.