Getting a tent back into its bag isn't difficult, once you know a few tricks.
The following steps show how to efficiently pack a tent and all its components.

The photos shows what we don't want. In this case the fly separate to the rest of the tent, taking up extra space and everything else stuffed into the bag. This is how it was delivered to me from a friend of mine.

Step 1: Packing Up the Fly

Straighten out the fly and then fold it lengthwise so that the outside (the bits with the guy ropes) is inside the fold as demonstrated in the second picture.

With a bit of practice and two people it is really easy to sweep the fly off the erected tent (after removing/undoing all supports) and end up with it folded lengthwise.

Step 2: Folding the Fly

Fold each end so that they touch the closest supports and then continue to fold in toward the center. Once the center is reached fold it over once more.

If this step is done correctly, the fly should be the same width as any of the folded poles, as shown in the last picture.

Step 3: Rolling Up the Fly

Starting at the top of the fly (the thinnest part and the bit that is in the center of the roof) roll it up tightly and then tie it up.
The tighter the fly is rolled the easier it will be to get it into the bag at the end.

Step 4: The Main Tent

The main tent has been laid out as if it had been erected. The front door is at the top of the picture, closest to the bins.
Ensure all zips are done up, including the windows.

Step 5: Adjusting the Roof

Using the center roof mounts, being clips or tubes for the support rods, pull the canopy so that it lays flat and isn't too bunched up.
Make sure the front door is flat, with the excess laying over the rear door, as shown in the second photo.

If you have a dome tent without a vestibule, work in a circular direction and pull each side tight before finishing with the front.

It really helps to have the tent still pegged at this stage.

Step 6: Folding the Vestibule

If your tent has a front or rear vestibule, fold them over so that they lay flat over the main body of the tent.
Make sure the front one is on top.

Step 7: Folding the Tent

Fold each side into the middle of the tent and then fold it over so that it is a quarter of its original width.

Step 8: Putting It All Together

Lay everything out at the front of the tent with the fly closest, then poles, pegs and any other part farthest away.
First completely roll in the fly
Add the poles and roll again (a half turn works here to keep it in place)
Add the pegs and roll again
Add anything else and roll up to the end and tie.

It is important to roll the tent tightly, if there is too much air once rolled it wont fit in the bag.