Just to confirm what the others said.
I went the cheapo route and put in thicker oil, which does help a little, but eventually decided to splash out on some springs and it's chalk and cheese. At 13stone plus, and with a pillion to boot, you really do need more spring in there.
I was going to fit 0.8kg/mm springs, from the various calculators available and my weight (
http://www.racetech.com/ ), but the place I went to only had 0.85 in stock, so I went with that (
http://www.k-tech.uk.com/ ). They're not
too hard for me (12st), and would probably suit your needs fine. The stock springs are 0.706 IIRC.
It'll be the best £70 you spend on it!