How to choose which acting credits to put on your CV
Deciding which credits to put on your CV is a luxury. Some people don't have many credits and some actors have credits that they'd rather forget for whatever reason - both are understandable.
Initially you want to put all of your credits on your CV, but eventually keeping it to a single A4 page is going to get tricky. You have to be completely objective - just because it was your favourite project to work on and the director was really nice, doesn't mean that it instantly stands out on your CV.
When you need to start replacing some credits, a good place to start would be to take off some of the older credits, especially if they happen to be quite old. If you order your credits chronologically, that's pretty easy - take a couple from the bottom and add the new ones to the top.
However, it's not always advised to order them strictly chronologically. It's a good basis to start with but you have to draw attention to your CV and make it stand out. This means putting your biggest credits at the top for everyone to see straight away. You'll have a good idea of which are your biggest credits, but they might be quite old, so you'll have to put it into context of all of your other credits and weigh up the age of the big credit against the value of the credit.
Another aspect to help in your decision, is knowing what kind of actor you are. If you're now focussing on film and all of your latest credits are in films, then you can get rid of a couple of theatre credits you did ages ago. Obviously if you're going for theatre then drop some of the film stuff.
Overall, it's easier to make a decision when you have all of your credits together. From there you can weigh up the age of each credit, the value of each credit and the type of credit depending on which roles you're targeting now.
Don't forget it's best to be as impartial as possible and not be sentimental - collate all of your credits and ask friends/contacts in the industry to see what they think would benefit you most. You don't have to take their advice, but it's always nice to bounce ideas off other people and get a different point of view.