I don't do a lot of trim work, but when I do, it's often stain-grade or pre-painted, and the miters have to be perfect! I hang doors, mostly, so I rarely have a full-size work table set up, just a door bench. Most of the casing I work with is small. At first, I thought it was too small to survive a Clam Clamp, but I've learned better (more on that later!). So I came up with a method for pre-assembling casing that didn't require a large table but guaranteed tight miters. There's no staples, no biscuits, just glue. But the pre-assembled frames are so strong you can carry them in one hand.

I made several sets of pre-assembly jigs, which are perfectly square and have a thin strip bordering two edges. I start by clamping the legs to two of the jigs with A-clamps. A single sawhorse is all I need for this technique. The back of each jig is covered with a non-slip material so the jigs won't slide off my saw horse (available at home centers: Solid Grip Liner, Contact brand $4.95 for a 18" x 48" roll).
Next I glue up the miters with Titebond.
Then I place a large spring clamp on each corner. These clamps are similar to the old Ulmia clamps. They apply a lot more pressure than Collins Spring Clamps, but the pressure is all right on the miter. They leave a single hole in the back of the casing, which is easy to fill.
On wood moldings I nail the corners with a brad, but on mdf moldings, I let the glue do all the work--mdf casing splits unless you nail it with a 23ga pin.
The assembly jigs do all the work. They keep the pieces square, and the A-Clamps help hold the casing flat, so the miters come out perfectly . I made several sets of these assembly tables.
With several assembly jigs, I can leave a frame to dry while assembling another one. As soon as I finish a frame, I pick it up off the sawhorse, carrying it by the assembly jigs. It only takes a few clamps to stay busy.
The glue sets in about ten minutes. By that time, you can carry the frame in one hand and even shake it and the miters won't open!
	.	I sent the pictures you just looked at to Gary about two months ago (that's how long it's taken him to put them on his website!). In that time, I've figured out a much better way to preassemble casing, especially small casing. Now I use a Clam Clamp
To use Clam Clamps (see Gary's tool review) on thin casing, I rabbet the assembly tables. For typical 9/16 casing or 5/8 thick casing, I make the rabbet 5/32 in. deep--that centers the clamp pins perfectly on the casing. Because the casing is so small, I remove the two lower pins, otherwise the Clam Clamp puts too much pressure below the miter.
This technique works much better wtih Clamp Clamps, but the process is the same: Always clamp the casing to the assembly tables first. I position the casing so that it hangs over the rabbet just a bit.
Once the Clam Clamps are on, the assembly tables make it much easier to move the frame. You can't get this kind of clamping pressure from spring clamps, and when you use Clam Clamps, the joints are so durable, firing a brad nail through the miter becomes a waste of effort.