I have a form that displays queue of messages and number these messages can be changed. Really I want to "indicate" label (queue length) when the number of messages were increased to improve user experience. What is the best way, according to user usability, to attract attention to this label? In additional, users get a lot of messages per working time, so it should be is not so busily.
In this case, I would probably update the count periodically, without any animation to draw attention to it. The user will get used to the number changing and will look for it when they are ready. It's usually not a good idea to interrupt a user's thought process without good reason.
If you want to draw more attention to it, and you've come up with good reasons for it, then I would use an approach similar to the Scriptaculous "highlight" effect. It briefly flashes the container and settles back down to normal. It is very important that you don't employ this too often or you will make your users sick. In short, do not flash/shake/etc. more than three times a second. For something like this, I would say don't even update more than three times a minute.
The term what you are looking for is spotlight effect: