How to find the right agent
In this industry, an agent who is right for one person might not be as effective for someone else. This can be due to all kinds of reasons: age, experience, gender, training, or if an agent already has someone who looks similar to you on their books. The question is, though, how do you overcome this?
Some agents specialize in certain types of actors, for example commercial actors, or child actors. It can be difficult to decide whether to seek representation from one of these specialized agencies, or one that takes on a wide variety of actors. Taking my example of agencies for child actors (which can include young performers up to the age of 21 in some cases), I personally think that it is more beneficial to be represented by one of these rather than a larger agency who may have a few young people on their books. This is because casting directors will go to the agencies who deal with solely children first, as they will have a wider selection of actors to choose from. However, certain agencies for children may be more geared towards smaller children, leaving older children with less work. Similarly, the majority of the work some agents put their clients forward for might be more model-based, whereas others will be more focused on acting. In order to find this out, it is possible to look in the 'news' section of agency websites to see what sort of work their artists get or, alternatively if they have a Twitter or Facebook page, have a look through this.
To find out if an agent is right for you, it is a good idea to look at the kind of people they have on their books. The top agencies like Curtis Brown and United Agents represent predominantly high-profile, experienced actors, so they are unlikely to take on many actors who are just starting out, unless you have a really unique skill or look. Even if you are offered representation by these kind of agents, it is unlikely that they will be totally suited to you because they will be more focused on putting their 'famous' actors forwards for a role, particularly if they are only able to suggest a limited number of actors for a single role. Therefore, actors who are less experienced will find that they are more suited to smaller agencies who can get to know them really well and so suggest them for jobs they are 100% suited to.
When watching dramas on television, it is a good idea to go on IMDB afterwards and find out who the actors were playing the roles with a similar playing age to you. Do a bit of research to see if this actor has been to drama school and how much experience they have. If you find that they have a similar background to you then look on Google to find their agent and perhaps consider applying to them.