You can improve the efficiency and coverage of your irrigation with a few minutes of time and just three tools. This week I'll be covering the basics on how I get a spray zone (fixed sprays, not rotor heads) adjusted.

First, grab a small flat-head screwdriver, hand pruners and a small shovel to straighten heads.
Walk to your controller and start the first zone of your sprinkler system.
As you're walking around the zone look for any leaks. Large leaks such as broken pvc laterals will cause the pressure in the whole zone to drop and will be pretty obvious (look for the geyser). But smaller leaks may go unnoticed for some time. The most common are worn heads that are leaking around the stem and are caused by worn out riser seals.
Check that each head is spraying where you want the water to go. Obvious, I know but there a many systems I've worked on where spray heads are spraying out into the street or turned and watering an area another zone is covering. Take a pair of channel lock pliers and grab the stem of the sprinkler and turn it (preferably clockwise) until the spray is pointed in the correct direction.
Check each head to see if the spray is partially blocked, there is no spray at all, heads are sitting too low (the nozzle is barely clearing soil/grass/mulch), spray is getting blocked by plantings or heads are crooked. Use flags, sticks, whatever you can find to mark each head that will need adjustment.
The spray should be a nice uniform fan, if there are gaps sometimes you can dislodge the offending particle with a small wire poker, if the nozzle is totally blocked (ie- no or little spray coming out) check the small set screw on top of the nozzle to see if it is screwed all the way down shutting off flow, if it is back the screw out until the nozzle starts spraying.
Now if the nozzle still isn't spraying it means that has a total blockage and needs to be replaced. Please note the numbers on top of the nozzle (ie- 8Q, 8H, 8F, 10Q, 10H, 10F, 12Q, 12 H, 12F, 15Q, 15H, 15F, SST, EST, CST, etc, etc) and replace it with the same nozzle. This is very important! If you replace it with a different nozzle you will either throw water where it shouldn't be (sidewalks, roads, onto your house) or you will not have full coverage and get brown spots in your turf. Your local irrigation supply house will have every nozzle you will need; Home Depot will have a pretty limited selection of nozzles.
If your nozzle was full of debris chances are that the head is sitting too low in the ground. The top cap of the sprinkler should be level with the top of mulch or 1/4-1/2 inch below the top of sod. Dig up the head until you have the whole head and a foot or so of the flex pipe below exposed and raise the head so it's sitting at the proper height and back-fill.
Once you've gone through and replaced nozzles, raised/straightened the heads turn the zone back on and check to make sure you haven't missed any heads and they are all spraying in the correct direction.
If there are heads that are buried in shrubs/perennials you can either prune some leaves/branches out of the way or if the head is really buried it may have to be moved out from behind the plant.
Finally now that all the sprinkler heads have been straightened, moved, re-nozzled restart the zone from the controller and check to see if you're getting good coverage. For lawn areas and areas with thick ground-covers this means total coverage, ie all spaces should be getting hit by at least 2 sprinklers. Shrubs and perennials are a little more forgiving in that every square inch doesn't necessarily need to be getting hit, but each plant will need watering.
Coverage issues take a bit more expertise and I would recommend that you contact a professional to take a look at your system if the above steps don't get you to the point where you have good coverage. There are many variables that could be causing a lack of coverage than a simple nozzle change won't fix at times from a change in water supply pressure, faulty valves, poor system design, or nozzles having been swapped out to ones with a higher flow causing a pressure drop.
Congratulations! You've finished tuning your first zone, now move on to the next zone of spray heads and repeat!

FAQs

Basic sprinkler system maintenance, part 1 - adjusting pop-up spray heads.