Do you self-promote or do you find an agent to promote you? Getting an Agent would seem to be the easiest answer, but an Agent may be promoting a dozen or more people - who's to say you'll be the focus of their attention?
Some say do the job you do well and let other's do the jobs they do. If you're a software engineer, rebuilding a car engine isn't something you should attempt if you need the car engine to run. If you look into rebuilding car engines as a hobby, or curiosity - and there is no need for the engine to actually function when you're done - great! But, if you need the car tomorrow, hire a mechanic. Is this true for agents as well?
Certainly the world of Classical Music is vast; finding the right person to talk to in order to get a piece performed is a skill all of it's own. Spending time trolling the web, making phone calls (sending emails) and reading the stacks of magazines about who's doing what where isn't necessarily the most effective use of one's talents. Numerous composers have personal websites - and that is certainly a start, but just having a website doesn't mean anyone ever sees it. The chances of someone who might want to perform your work randomly finding your site is virtually non-existent if you don't have some means of posting the URL in places that might garner your site some attention.
I have found many composers through their comments on blogs (their sites are linked to their names). Ok, so posting onto blogs is one way of getting attention - but that means you'd better have something worthwhile to say. Random comments just to promote a blog only make the author sound like an idiot which tends to send people in the other direction.
I've seen a couple of ads on Google for composer's websites (popping up because I'm searching for Classical Music or some such thing and their engine says an ad for a composer would be a good thing to show me). It sort of works - ie., I followed the link (more out of curiosity than anything else) - but found in three cases that the linked sites were no longer functional. Well, advertising is good if it works. Google's engine targets the audience, so this might seem like a good option. But if by the time people find you your site is no longer running than the ad didn't really work.
There are books that list Who's Who in the music world - a list of some 10k names with addresses and the like. This might sound like a great tool for contacting people - and if you know who you're looking for, yes, it is a real help. What it doesn't tell you is if they are someone who might be interested in your work. So what if you have an address or phone number... If you call someone out of the blue are they really going to talk to you? Maybe (if you have a good phone personality - but that gets back to specific talents - and I'm not sure I trust my phone personality to get me past the "Hello, this is Chip Michael..."
Music Publishers (like Boosey-Hawkes) will do some market flogging of your works, but like agents, they will spend the bulk of their time on people who bring in the most money. So, if you're like me, an unknown, getting more than just a listing is rare.
What about composer forums or artist collectives? The Scottish Music Centre attempts to promote Scottish music, and if you're looking for New Scottish music this might be a good source. However, Craig Armstrong (film composer) has only one score listed and that's from 1988, so either he feels he has outgrown the organisation or it wasn't worth his time. James MacMillan, on the other hand, has 41 listings; it obviously seems to work for him. The lesson here is to know your audience and where they are likely to go to find you (or where they look to find material you might be able to provide).
Some of the more prominent collectives are ASCAP (American Society of Composers, Authors and Publishers) and BMI (Broadcast Music, Inc). Both of these are huge organisations, but fairly specialised. While there are some things they both do, there are some things one is better at than the other, so you really have to know what it is you're trying to achieve when you contact these organisations. Bands that write their own stuff tend to go with BMI, while (of the people I've talked to) composers tend to favour ASCAP.
There are other organisations/collectives out there. The American Composers Orchestra comes up pretty prominently on searches, as does the American Composers Forum and the Music Publishers' Association. There are literally hundred of others, some that cater to composer's from a specific state or region (Australia born composers verses UK born composers), others that focus on a style of music. Each one is trying to fill their niche, and if their niche is your niche, it just may be a good fit.
If you're looking to get into Film scoring there are organisations that try and pool resources, provide a central location for job postings to allow artists in one field find artists in another. Talent Circle is a collective for people in the UK film industry and Mandy is for the world film industry at large.
On the other hand, finding the right agent can be a chore in and of itself. James MacMillan and Peter Maxwell Davies (a pair of Scottish Composers) are members of intermusica. Hot House Music works out of London and represents people like Hans Zimmer. Film Music Network provides a service of matching composers with agents - sounds nice, but you have to join their network to have it work for you. Taxi is another network which provides a similar service, but is extended to musicians of all sorts. If what you want to do is get into film scoring, Film Score Monthly has a nice article with some advice from agent Richard Kraft about attempting to becoming a film composer - and finding an agent which is anything but encouraging (a good read, but it isn't meant to get your hopes up).
What about posting your music on other websites? There are a host of sites where you can post your music, either in audio form or in midi. Most of these cater to older (out of copyright) compositions/composers - but some do post current composers works if posted by the composer. Classical Archives is one of my favourites for a source of material. However, if you post your music here you are allowing anyone to download it for free.
I don't have an answer for myself as yet. I think I am probably at the stage I need an agent if I am to go any further. But it is rather like taking a plunge in to a cold pool - you just have to do it. However, depending on how the concert goes, I think a agent is the next step for me. A decent webpage is also on the list (more than just this blog or my music website - which desperately needs work). And then there are all those compositions I'm eager to get started on...