Your portfolio is the single most important display of your work. A bad portfolio will reduce any chance you have of getting a job. Your resume may talk about your talents, but your portfolio will do the talking! Your portfolio must prove your resume to be true and show only the most outstanding pieces you've created.
Limit your samples, do not include everything you have ever created. Most employers or clients do not want to spend all day looking and talking about every sample displayed. Make sure and point out your most important designs and leave the others behind. However, do not limit yourself to only a few samples. Make sure your portfolio has body and shows your diverse abilities.
Make sure and place your best designs toward the front of the portfolio. If you have big clients, make sure and put those samples closer to the front as well. As you continue through the interview, the employer will start talking more to you and looking less at the portfolio. You must get their attention early by moving your samples. Try to keep your samples in order of print, web, ads, etc..
Some designers will create great looking portfolios while others will make too much of it. For example, you should consider keeping the layout of your pages clean and simple, allowing for the artwork to do the talking. Use black or other solid colored papers to mount your artwork on. Keep 1 to 2 items per page as not to distract from the creative. Some designers on the other hand will make a portfolio busy by adding design to the pages along with the creative sample that they are displaying. In some cases, this may be effective, but for the most part its distracting to the original creative. Remember that you're showing who you are through your designs let them do the talking for you both.
Last but not least, is leaving your portfolio for a prospective client or employer to review a good or bad idea? This is a bad idea I have known too many portfolios getting lost and lawsuits coming about after that. If you feel you have a chance and can trust the client or employer, then leave your portfolio for no more than one day for them to review. My suggestion is to leave a sample package or direct them to a Web site. A Web site is the best way to let everyone review your artwork at any time. If they are serious about seeing you after that, then you can bring in your actual portfolio. I personally like to see both, but I would rather look at a Web site up front. Make sure to include a Web address on your resume. Also, if you do leave your portfolio somewhere, have some form of identification on the outside, either a luggage tag or your logo, as well as a label on the inside with your contact information. This will help keep your portfolio from getting lost.