Simple and easy?
Buy a truing stand, a dishing tool and tensiometer and then take a wheel building class. After that, practice and practice and practice building and fixing wheels.
After building several dozen wheelsets and fixing and truing my own wheels, I'm no where near knowledgeable to try anything beyond a standard 3X lacing pattern. Unless you're Jobst Brandt, I don't think it ever becomes simple and easy.
Your problem sounds like improper spoke tension.
As has been correctly implied the ART of wheel trueing is not just twisting a few nipples (no rude jokes here...save 'em fer later). There are many factors that influence the stability of a wheel. If the basic tension (dramatically different on each side on a rear) is not in the ballpark the wheel cannot true well or hold its true.
If you are not trial riding or racing on very uneven surfaces a good true should last a very long time (of course time is relative). My own wheels on the 3 bikes I ride regularly have gone hundreds of miles between needing a tweaking unless I hit a curb hard or twist the tire in a bad bike rack.
Check your techinque and read up on the techniques needed for good tension, dishing and trueing to be sure you are approaching the skill with the right knowledge.
If you're going to do it in the frame, taking the tire off will help. It will allow you to see what you're doing, especially if you don't have narrow road tires. Will also allow you to check out of roundness, or "hop".
If the wheel keeps loosening up, you aren't getting enough spoke tension in it. The dish may be way off if it's happened several times. Make sure the wheel is in the dropouts evenly, then compare the distance from the rim to each chain stay. Because of the angle of pull of the spokes, it's much easier to pull the rim to the left than the right once the tension is getting close to optimal. So err in the direction of having the rim closer to the right stay than the left until you're nearly done. Then go around all the left side spokes and give them a quarter turn each at a time until you pull the rim back to center.
You can adjust the hop out using 2 popsickle sticks as a guage. Place one on top and one below the seatstays and tighten a bread twist wire around them near each stay. You can then slide the sticks down next to the rim to check the hop. Find the high spots and tighten the spoke on each side of the wheel nearest the high spot. The same amount on each side's spoke. Do a couple of rounds working on the high spots, then a couple of rounds working on lateral trueness. Keep alternating until both are reasonable.
It will help a lot to have a properly tensioned wheel close at hand, preferably one that has a known history of staying true and tensioned for many miles. It should be the same diameter with the same gauge spokes and lacing pattern. You can pluck the spokes on each side, comparing the tone to the wheel that you're working on. Build tension as you true your wheel, while doing what you can to make all the spokes (on a given side of the wheel) make the same tone when plucked. Once you have the wheel reasonably true, you can bring the spokes up to proper tension. Go around the wheel giving the right hand spokes each a half turn. Then give the left spokes all a quarter turn. Check the rim's lateral position to the stays. If the rim moves appreciably toward one side, change the amounts on the next round to compensate, or go around just one side with a quarter turn each.
The goal is obviously to have the wheel round, laterally true, with the rim centered in the frame, and each side's spokes having the same, and proper, tension. In practice, there will be a couple spokes that have a bit more tension, and a couple that have a bit less. If you're careful and patient, you should be able to make your wheel run for hundreds of miles, if not more.
If you're able to actually get the wheel true, your problem is lack of proper tension. Marks' reply above is correct. The only problem you'll run into is that you'll need a rear wheel with the exact same number of spokes, same diameter, same spoke thickness.
The tension of the drive side (spokes run off the side of the hub directly behind the cassette) will be approximately double the tension of the non-drive side spokes. This is for a rear wheel only. A front wheel will have equal spoke tension on both sides.
If you enjoy working on your bike, it might be worth the $50-$60 or so to get a park tool tensiometer.
Drop it off at a bike shop.
Spend $30 ....help a mechanic pay rent .
I know this is an old discussion, but I have the same problem right now- it seems my rear wheel goes out of true every few weeks or so. I am a daily rider, at least 16-20 miles a day, I carry panniers on the back rack and go grocery shopping with this bike, too, so it is many times pretty heavily loaded. I know the wheels are stock, so not the best which I understand may be the problem. The rear wheel was so bad today that it was rubbing the brakes (this is after I loosened the brake cable enough to allow the wheel to spin so I could at least commute in to work-mind you, the wheel was okay yesterday, it just is getting progressively worse). My question is: Should I just spring for a new wheel; have the existing wheel re-built; or should I just keep taking it to be trued at the shop again (3rd wheel true in 2-3 months) or finally, should I just find another shop to have the wheel trued and tensioned at since I have explained this issue before and they happily true the wheel and let me go on my merry way, only to see me in a month or two with the same issue?
I have no real problems with the front wheel, it rarely has gone out of true, it is just the rear which I can only believe is from my grocery store trips and other loads on the back rack. Any advice is appreciated- thanks.
Try truing your existing wheel and install a front wheel rack with the panniers. This way the cargo load will be more evenly distributed (especially since the rider's weight is more on the rear than the front wheel) .Works for me, and I have cheapo stock wheels that rarely go out of true riding about 800-1000 miles a month. If you can't find a front rack at your LBS, here's a good one for $32.50. This will work on a 26" (MTB) or road (700c and 27") as well.