A portfolio should showcase the type of work you want to do, not so much the type of work you have done and are no longer doing. It should be narrowly focused to show that the model can be very good at modeling in a few related genres. A good combination might be fashion, beauty, commercial, and swim wear but such a portfolio should not include nudes, trashy lingerie, bondage, or goth. There is no reason a model can't create multiple portfolios targeted for specific genres. Some models will argue that they want to put in a little of everything to showcase their versatility. That model probably works better for models targeting photographers as clients. Models who want to target agencies, casting directors, and commercial clients should show only their strongest images in specific categories. Any weak images will weaken the entire portfolio.
Examples of bad model's portfolio include:
1. Same outfit in different poses.
2. Different outfit in the same location.
3. All pictures done by one or two photographer.
4. Unretouched images showing flaws.
5. Fake tear sheets.
6. Poorly shot photos of model in no name runway fashion shows.
7. Personal snapshots such as photos from a wedding or any photos which include non models.
Examples of good model's portfolio for agency models:
1. Beauty shot.
2. Natural light shots.
3. Full length fashion shot in front of seamless paper.
4. Swimsuit shot but not one of the model standing in the woods miles from water.
5. Tear sheets but not webzines. If no one has heard of it, don't use it.
6. Very light makeup shot or no makeup shot if the model can pull it off.
7. At least one picture pairing model with another model but not lesbian girl on girl shots the photographer suggested.
8. Editorial shots.
9. Sexy but tasteful lingerie shots. Think La Perla, not Frederick's of Hollywood.
For commercial portfolios:
1. A very strong head shot.
2. Shots showing model interacting or playing a role.
3. Shots showing model conveying a mood or message.
4. Interaction shots with other models or objects.
Most model portfolios are 9x12 while most photographers portfolios are 11x14.
Read more and contribute at Modeling Wiki.