Critically look at the work. Determine which pieces are the strongest, and why. Make sure that the pieces you choose will best represent you to someone who knows nothing about you or your work. Do not try to show how much growth has taken place, choose only the best. Artists may want to show a series of works as their portfolio, instead of individual pieces.
Keep in mind the audience. If you are applying to a themed show at a gallery, or trying to get into a specific program, your portfolio should show work that is relevant. Think of how your work will look in the gallery space, or, if you are applying to a program or residency, think of how will the work translate and relate to the program.
Ask your peers, colleagues, professors, or anyone else you respect for their opinions. Being objective about your own work is difficult. Other artists will be able to tell you if the pieces chosen best represent you.
Get pictures of the work. It is extremely important that the photos in your portfolio are professional. The pictures must be high quality, and they must not be altered in any way. If you cannot do this on your own, hire a photographer. For example, a drawing may be easy to get an accurate photo, but a glass sculpture may be nearly impossible to photograph. Spend the money; it's not worth jeopardizing your chances.
Create a digital slide show of your work -- this is your portfolio. Pieces relevant to each other should be organized together. Most organizations and schools accept digital format portfolios. This is more economical than slides, which is great if you've already spent the money hiring a photographer. Make sure your photo files are small enough to open quickly. If it takes too long to open, they will not be looked at. Jpeg is typically the accepted format. Most professors recommend a size of 800 pixels. Burn your slideshow to a disk.
Make sure you include your cirriculum vitae and a strong artist statement with your portfolio.