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Hope someone can help with this....I am using a Nikon D80 and 18-200mm lens.
If I try to take a self portrait either on self timer or using remote the focussing never seems to come out. Even if I focus on the chair I am going to sit on it is still always a bit blurry. Any suggestions how to get the focussing spot on ?

Are you using the AF or focusing manually? I bet manual will work better, since you are dealing with a set piece. Also are you able to use a mirror lock up or shutter delay? The blur you see may actually be mirror slap, not focus error. The Nikon mirrors are said to release hard enough to shake many tripods. I don't know about the D-80, but many Nikon DSLRs have a setting in the menus that will delay the shutter release by 4/10 sec to allow the mirror to trip up and seat. My D-200 has it in "pencil", opt d5.... Second option is mirror lock up, which on the D-200 is located on the left side on the single shot/burst dial, it's indicated by a Mup mark, and delays shutter release, by using a second press of the shutter. You press once and the mirror trips up, press twice or wait 30 sec and the shutter fires.

Would definitely opt for Manual Focus. Also recommend using smaller f-stops. The closer you are to your subject the shallower is your depth of field also the greater your degree of magnification (higher mm setting) the shallower is your depth of field. Smaller f-stops will always increase the zone of sharp focus.
If you are using Manual Focus and depending on the little green light to help you determine the exact point of focus, be aware that autofocus by its very nature is not entirely accurate. If you are still having trouble, try to determine if you are focusing in front of or behind the exact point of focus you're looking for and adjust adjust focus accordingly. Focusing screens today get you close but don't always win a prize!
All the above regarding the steadying of your camera at the moment of exposure are good points.

Manual focus should work, but auto focus should work well too. Get it out of AF-C mode. AF-C will take a picture regardless if it is in focus or not. Put the little focus mode switch below the lens to AF-S mode. AF-S mode will not take a picture unless it is in focus (AF-C tries to focus, but focus may not be as fast as the shutter... AF-C is NOT for still subjects). Then use the Remote, perhaps with the Remote's two second delay to be able to hide it from view. Pay attention to where your focus zone is in the frame. It should be moved higher, to be at the face area. Then you may have to move your head around as you sit there and click the remote, to find the place where your face is in the focus area. That is the only place where you can trip the shutter, when it is in focus. But you learn this place real quick, and you may want to move your focus zone to match where your face is.

Thanks for all the comments folks, I'll try out some of your suggestions.