How to build a high-quality acting portfolio
The first thing to ask is what constitutes as high quality. For instance, if you want to work in action movies then your profile needs to be catered towards that and will look very different from that of a musical theatre performer or a classical actor. If you think about the places you are aiming to work then you can get a better idea of the things you want on your CV.
When you're starting out it's good to get as many credits on your CV as possible, and list as many skills, accents as you can do. When your career progresses then you can start to filter out the smaller bits and cater your CV towards the areas of the industry that appeal most to you, as well as prioritising the work you are most proud of, or (for marketing purposes) the most prestigious and successful work. If you can look at every item on your CV and find a good reason why it's on their that matches the rest of your credits, then you're heading in the right direction. Don't credit the same company too many times. This is all objective however, some casting directors like to see that you're willing to work, so the more work the better, but some are only interested in quality and not quantity. So it's good to find a balance, or if there are particular people you'd like to impress, find out what they like and go with that.
In order to do this you first have to do the work, and this is a completely different question all together. However, on a general note, if you want to work in Shakespeare then get some Shakespeare credits. I knew a very successful West End musical theatre star who wanted to work with a certain producer on a certain Shakespeare play later on that year. This is where I met him (in an averagely small role) in the fringe, doing a Shakespeare play with a company in it's first ever production. He said it didn't matter where he'd done it, because for him at the time what was important was the fact that he'd performed Shakespeare recently. That was enough for what he was aiming to achieve at the time.
Once you feel you have enough credits then you need to think about which direction you want to go in marketing wise. You can either be the actor who goes all out, who has the website, the showreel, the personal blog, Instagram and Twitter links. You can even make yourself a fan page on Facebook. If you're going to do these things then really do them, and be clear and thoughtful with the design for your portfolio, whether it's a website or CV. Make sure there's a reason for everything and although you are being personal with this, remember that you're still a brand. No one wants to see your drunken nights out or emotional rants if they are searching for you as a potential employer.
You can also go the other way and have a very simple marketing plan, whether this be just your Spotlight, Casting Call Pro and agent's website, you might throw in a Twitter account for good measure. Different things work for different people, but be precise and be clear how you want to market yourself.
This same thing goes for your portfolio. You don't need to include everything. Keep onyl the best of the best. Choose pictures that are different and try not to repeat black and white/colour or the same looks for too long. For films, this is what showreels are for. If there's something which, for whatever reason, you couldn't include in your showreel then there's no reason why you can't include that too, but unless you are a film maker no one wants to sift through 15 short films when they're researching you, so only include the very best bits. Again biographies should not be too long, just give the clearest introduction to yourself possible and let the work speak for itself.
Think about your unique selling point. What makes you more interesting than other actors of your casting? Perhaps you play the accordion or you're a skilled acrobat, these things are good for promoting on your website, along with reviews and recent work. Keep the website up to date and fresh, but again, simplicity is the key.