If it looks good in Photoshop and Lightroom, but looks bad on the web, the most likely problem is probably either improper resizing resulting in artifacts or compression artifacts resulting from too low of a jpeg quality. To maintain the highest possible quality, try reducing using multiples of the pixel count for the original image so that new pixels don't have to be created. If you can't do this, play around with the different scalers. Bicubic is usually decent.

Another possible problem may be that you are using too low of a jpeg quality. Depending on the image, you can still see artifacts in to fairly high quality level jpegs. I normally use at least a quality setting of 90 when doing portfolio stuff. Max quality may be preferable for some images.

There could also be a color space mismatch potentially since Lightroom and Photoshop can work with color profiles where as most browsers won't, but if you are specifically using non-standard color profiles, you'd probably know about that already, so it's less likely.