Concert promoters make a fortune doing this for a living so certainly you can't expect my humble little price of the internet to be able to tell all, but I can offer some tips and ideas.
I used to promote seminars, not really concerts but when I helped a friends band get some publicity and promotion it seemed to work.
I got some new tips from a book and they seemed to help to.
First - think about who your target audience is.
What other concerts / gigs / events would they go to? We gave out flyers at other gigs in the area where other bands in the same musical style played.
Hence our target audience was there and an easy target.
One of the bands even announced our concert for us at the end of the set.
Second - flyers.
Okay, I should have made this the first point - I was just testing you!.
Try to put them up / hand them out where your target audience will see them (take a hint from the first tip) and make sure you don't just litter the place and aren't posting them anywhere illegal.
Make sure they will stand out, so colour is good, but expensive.
You will be able to get many more leaflets / flyers out there if you buy black and white.
It may be an idea to go to the local college and see if you can get a graphic design student to design a leaflet for you in exchange for free tickets.
Thirdly - get on the web.
Myspace has a great music section, and it can't hurt to promote your concert here.
Maybe link to a YouTube video of your band playing, or put clips from your last performance if you have a video of the event.
Fourthly - most local papers have a what's on guide or events page.
Make sure you have a listing here which is often free.
Is your concert newsworthy in any way? Try contacting the paper to see if they will do a write up.
Are you playing for a charity or worthy event? Are there any unusual circumstances around the gig? On a slow news day the paper may just get your photo in the paper and give a small write up.
Try getting a free mention on the radio as well.
Fifth - Alice Copper was played in London.
To promote the gig he had a big truck drive around with an advertising board on the back.
The truck "happened" to break down right in the middle of London, and caused traffic jams that made the news.
People in the UK saw a big picture of Alice Cooper right on there TV screen, and soon everybody knew Alice was playing a concert in the UK.
Now, I don't suggest you do this, but be creative.
What about putting stickers promoting your event on coins, pasting flyers to your vehicle and leaving it parked in a busy car park where lots of people will see it.
Try putting stickers on any mail you send out - you'd probably be surprised how many pairs of hands that mail goes through! I think that is enough for know to wet your appetite.
As a final thought, think about your budget.
It is very easy to spend a lot of money promoting a concert if you don't know how.
I've learned from some of my mistakes which is why I bought the book - I try to learn from other people's mistakes these days.
If you really want to know how to promote a concert and get the full range of techniques I suggest taking a look at the book as well.