It doesn't matter if you are putting together a Web designer portfolio or one for graphic design, there is one key rule to keep in mind--select only your best work for inclusion. When it comes to showing your abilities, keep in mind that quality is always better than quantity. This goes for on-line work in addition to print samples.
So the next step raises a question--how do you choose your best work? Try and think about several things when selecting things to include. For example, if you have worked for a particularly well-known company, you may want to include them in your portfolio. Also, try to look at the work from a technical point of view. Is it outstanding? If not, you may want to leave it out. Even just a few well-chosen pieces can show your ability.
Another thing that can be helpful is to get a second set of eyes on the work you are considering for the portfolio. Choose someone who has technical expertise as well as an understanding of the field, and have them take a look at your work. The feedback they give may help you finalize what you include. Once you have made your final selections, you are then ready to assemble the portfolio and shop it around.