I've had my SV650 since 2005, and I have NEVER even checked the sag. I weigh about 155lbs and I just set the preload to 5 in the rear and left it there.
Well yesterday I put a '07 GSX-R 1000 shock on the back. Nothing I remember reading said that it was about half an inch shorter, but that's a side point.
I took the bike out for a short ride, and I like the feel of the stiffer rear spring for the sweeping corners. It's hell on a bumpy road though. I'm going to slowly tweak the high speed compression damping to see if I can smooth that out a little.
On to the sag: Not that I don't have friends, but I usually prefer to work on my motorcycle alone. Measuring the sag on the motorcycle is kind of hard when you have to sit on the thing while you're taking the measurement. This is what I did.
I used a yard stick, two 12 inch rulers (with metric) and some masking tape.
I taped one of the rulers on the side of the tail section with about a 2 inch gap between the pieces of tape. This two inch gap should line up above the rear axle. The yard stick goes behind this ruler between the tape and rests on the rear axle nut. The cotter pin through my axle just so happens to be in a stop to hold the yard stick in place. Here's an image:
Some may note that there is nothing protecting the paint. This is because the paint on my bike is in bad shape and I plan on repainting it soon.
Then I marked the location where the horizontal ruler crosses the yard stick, this is the initial position and will be the mark you are measuring to with the second ruler.
Next, all you have to do is sit down on the bike. I put the kickstand up, and applied the front break. I tried to make sure that I wasn't bouncing the bike when I got on so I would get as accurate a reading as possible.
The line with the circle at the end is the initial position of the yard stick so all you have to do is measure from the edge of the ruler to the initial mark. Presto, one man sag measurement!
Thinking about it later, I probably should have made the initial mark with the bike standing vertically, but with this mark, and the GSX-R shock, I'm only able to get 25 mm of sag, which seems decent. I plan on getting some shorter dog bones to raise the height back to stock: I can touch flat foot now and I don't actually like it.
I hope this helps people out!