There are basically three types of frames for large wall tents, and they each have their idiosyncracies. Let's start with the simplest ... a center pole (or, in the case of a wedge or "bell" type, two center poles) and perhaps a ring, but no side poles. Slackness in these tents is almost always due to a center-pole that's too short.
"How can it be too short?" I hear you ask. "It was fine the last time I set up!" Well, that was then, and this is now. Over the years, the fabric might have stretched to the point where you now have a tent that's slightly bigger than the one you purchased. Or the center pole may be too short because it's sitting in a slight depression in the ground ... or the entire tent site might be slightly concave. No matter how hard you pull on the perimeter of the tent at the bottom, you're not going to make that slackness go away. The cure is to carry a few little squares of wood to use as shims. Make them out of 1" pine (which is actually 3/4" thick) or 3/4" plywood (which really is 3/4" thick). Mine are about 4" square, but just about any dimension works as long as it's bigger than the base of the pole. The advantage of bigger pieces is that you can place them where you want your pole to be and then stand on the edges of the wood with your two feet, thereby holding the shim in place as you lift on the center pole and reposition it over the shim. On longer camping events, I've sometimes found it necessary to start with one shim (for reasons I'll explain shortly) and add another shim or two as the days go by, to adjust for the fabric gradually stretching under load.
I often start with one shim because I usually make my center poles a teeny bit short. If I'm setting up on an area which is slightly domed, instead of concave, a center-pole of exactly the right length would actually end up being a little too long. This can cause slack, too, in some designs. To understand why, try this little thought experiment. Start with a simple cone tent with one center pole. We'll mentally pull all the stakes out at the bottom. Now we'll put them all back in again, this time in a circle that's only half the circumference of the last one. Now when we raise the center pole as high as it will go, we'll have tension in those parts of the cloth that go from the stakes to the apex of the tent, but everything between the stakes will be sloppy-loose. That's the same sort of uneven tension pattern you get with a center-pole that's too long. So to make everything nice and tight, you'll have to re-set all those perimeter stakes to the greatest possible circumference, so that the skirt will be tensioned all the way around, and then raise the center pole last.
When all of this is being done, the guy ropes (if there are any) should be slack. You want to be tensioning the fabric, not the ropes. Only when everything looks snug all the way around should you tighten the guy ropes, and then only enough to "lock in" the tension you've already achieved. If the tent is well engineered and its fabric is snug, it really doesn't need a lot of tension on the ropes to shrug off the wind.
A subset of this first category is the round pavilion that uses a ring for making the eave rigid (I'm using the word "eave" to denote the part of the tent where the conical roof stops and the more-or-less vertical wall begins). It's especially important here to start with the guy ropes slack. If these ropes are tight to begin with, it's quite possible to lift up on the center pole until the canopy is tight as a drum and still not be able to tension the sides. The solution is to loosen the ropes and lift on the center pole until all the tent is tensioned ... canopy, sides, and all ... and then put enough shims under the center pole to maintain that tension. Then go outside and tighten the ropes -- again, just enough to lock in the adjustment without putting any additional tension on the canopy.
The next category of tents includes those with side poles. For tension problems above the eave-line, everything in the previous paragraphs also applies to this style. If your canopy looks good and your sidewalls are too slack, please understand that it's the side poles that govern the tension of the sidewalls, and there's not too much you can do about it. If the tent has sloping sidewalls, check to make sure that the base is staked out properly. Most of the problems here consist of the tent's bottom being not being pulled out far enough, although uneven ground can play havoc here, too. If you're camping in cow pastures, where those critters leave little craters everywhere, you might have to bring along a few more shims to make everything even.
Also, if your sidewalls hang off a rope threaded through loops in the canopy, check to see if there isn't too much slack in that rope. You don't want so much tension in that rope that it "gathers" the base of the canopy, but you don't want it too loose, either. How can you tell when too much is too much? Adjust the rope before the sidewalls are hung. Start with it completely slack (as in "untied") When the skirt of the canopy is snug and tensioned, start adjusting the rope so as to remove all the slop from it without affecting the canopy skirt's tension. Tie it off there, reflecting that life is change, nothing is permanent, and you'll probably have to re-adjust it by and by.
The third type of tent uses a completely rigid frame, with the fabric taking almost no tension. Tension problems on this sort of tent are often the hardest to sort out, because they usually indicate a mismatch between the dimensions of the fabric and those of the frame. Curing them involves re-working the frame or the fabric (usually the frame). While it's true that the fabric isn't working as hard as it would in a true tension structure, it's still important that the fabric be snug ... not only for look's sake, but to increase the longevity of the tent, and to prevent the problem from getting worse. If the cover is loose, the wind luffs the fabric, the fabric weakens and stretches, becoming looser, so the wind can luff the fabric even more, which makes the fabric stretch even more ... and this stretch is more serious than the usual stretching you get with natural fabrics, because it's a sign not of the fibers accommodating themselves to climactic conditions but of the fibers giving up the fight entirely.
Since I mentioned stretching, I might as well mention that tents made entirely of synthetic fibers (of which Nylon is by far the greatest offender) can be very temperature unstable. They can shrink when warm and expand when cold. If the tent frame is made of steel, the steel will shrink when cold and expand when warm. Do we see a problem here? So a certain amount of slack may be unavoidable. But in windy conditions, the fabric will certainly be taking a beating; it will leak before its time and will eventually expire prematurely. If you want to save weight, you might consider using Nylon only for the sidewalls (wind speeds are lower closer to the ground, and sidewalls don't have to be quite as water-proof). It also helps to attach the fabric parts to the frame with elastic fasteners like bungees or shock cord, to allow the fabric to change dimension without becoming too slack.
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