A few final notes
JPEGs compress smooth images better than images with sharp edges, so you could try smoothing the image slightly before compression (your web graphics application may have a slider to apply this without affecting the original image). This gives a good reduction in file size for little change in quality of the compressed image.
JPEGs don't have to contain colour data. So if your image is made up of greys only, save it as a greyscale JPEG and save space by omitting the colour components you don't use.
If you know most people will be viewing your image on 256 colour screens, use a lower quality setting. The dithering applied to the image by the browser will remove much of the extra detail in this case anyway.
Just as with GIF files, creating small JPEG files is not something for which you can apply simple rules to give the best results. The lossy compression takes advantage of the properties of the human eye - so only the human eye can judge 'good' settings.
Also, each picture will be different, because the eye responds differently to every picture. But practice makes perfect. Just play with the sliders and see what comes out!
Ben Summers is a UK developer specialising in providing dynamic web solutions for interactive online applications and developing web graphics software. Ben's company Fluffy Cloudshas just launched a new web graphics optimisation program called Ignite, currently in public beta and available for download from the Ignite Web Site.