How do you maintain a good portfolio as designer for interview?

The best advice is to always be working and thinking about one's portfolio. Keep original digital files as well as hard copies of the very best design work. Digital portfolio's are common and widely used. Some employers also like to see sketch books, so keep up on sketching out ideas for new projects. Presentation is everything, so create a complete package. A nice case containing hard copies and explanations of your best work, along with a digital portfolio or slide show on a CD and well designed Resume and or Show Resume is the easiest to assemble. The more material one collects the easier it is to create portfolios that address a specific industry or style. To maintain a portfolio collect, collect, collect, make it a habit so when the time comes a superb portfolio can be assembled.

There are also great resources in the past work of other designers' self promotion pieces. The self promotion piece is a very good way to introduce yourself and your work to potential employers and, depending on the design, it can serve as a portfolio.