Zoom all the way out (so that it says 1:1 in the titlebar of PSP)

Go to the layers pallette (view, toolbars, layers) and click on "layer 2" to activate it 

Floodfill with a dark colour - make sure that it's totally different to the one you used for your background and it's not used around the outside of the image, for example, if the background was green and the outline of image was black, don't use green or black... use purple

Go to the layers pallette and click on "layer 1" to activate it 

Use your eraser tool at the following settings:

Round shape, size 10, hardness 100, opacity 100, step 25, density 100.  Don't build up

If there is any obvious background you've missed take it off now

Save your work

 

Zoom in twice

Use your eraser at the following settings:

Round shape, size 5, hardness 100, opacity 100, step 25, density 100.  Don't build up

take off any obvious background but make sure you don't go over the edges of the image

Save your work

Zoom in again as much as you need to for it to be comfortable to your eyes

Use your eraser tool at the following settings:

Round shape, size 1, hardness 100, opacity 100, step 25, density 100.  Don't build up

Take off any obvious background making sure you don't go over the edges

Save your work

Take another break away from the screen... it's always better to be safe than sorry!  :o)