How to have a successful acting career without an agent
An actor without an agent just needs to find the companies that are willing to talk to them without an agent. I know for a fact that Future & Secret Cinema have employed up to half a cast without agents before.
It's important if you represent yourself, though, to find find a balance between being a hard ball that is too awkward to employ, and being a pushover who gets underpaid and mistreated.
An actor without an agent can still use Spotlight, Casting Call and Equity to their advantage to find jobs.
Finally, when there is no work, make some, Write a one man show that is easy to put into a variety of spaces, enter it to Fringe Festivals (other than Edinburgh), have a look at Prague, Amsterdam or Bedford Fringe and at the end of that show, the profits are all yours. Fringe Festivals are a great place to get press and that can be used to book yourselves into smaller arts venues around the country. Venue's like Old Red Lion in Islington host visiting company nights on a Monday and for the cost of a venue and £40 of leaflet printing you have the potential to make a £600 profit. It's about taking risks, you need some money to make some money.
Get a website too and make sure your only presence is up to date so that you look proactive, once you have that, people will start to take you seriously.
Network whenever you can too, you can meet the most important people in a Charing Cross burrito bar or the back of a Soho pub, carry professionally printed business cards to sell yourself as serious and make sure these have an email, telephone number and link to a CV. You can also get some from www.moo.com that have your headshot on, helping people to put names and information to faces.
It might also help to keep your social media presence quite open. It's easy to remember someone's name if you can see it next to their Facebook photo