As a freelancer, I have bid for jobs on pretty much most of the free job boards simultaneously with absolutely no results. Since these boards rarely post the same jobs, you will not increase your chances of getting jobs by using multiple sites and more than if you exclusively used one board out applied to more jobs. It is a numbers game. The more jobs you apply for, the higher you chance of getting hired...but keep in mind that all of these boards also have thousands of freelancers. Anywhere between 10-15 bid on each and every job posted. To increase your chances of finding jobs, you need to either specialize in a niche field with less competition or use paid boards. It might cost you a bit up-front, but you will find the quality of leads to be much greater. I use Guru and Freelance Switch Job Board. I also make good use of craigslist for local jobs.

Absolutely.

I'm a freelance (in the UK) and when I'm 'on the market' I'd go to all the web sites I can think of (as well as contact companies, agencies, do networking etc)

At the end of the day, initially at least, no-body promises you anything, and - as David said - it's a numbers game - the more people you talk to, the more chances you have to secure a contract.

The key thing, I think, is to always make sure people know where you stand - on the initial contact with any one lead I will always ensure they understand I'm looking into other options as well (as I'm sure they are), but as things heat up you reach a point where you have to stop hedging you bets and make a commitment to one channel, at which time I let the main potential customer I'm now committing to be available, and letting the others know I'm no longer available.

should this not lead to a contract I do the reverse - let the person I'm talking to know I'm back to looking elsewhere as well, and go back to the boards/agencies/networks.

It's quite a bit of work, but it pays off big time, in my view, as it's respectable and professional. At the end of the day - life is mostly an exercise of matching expectations, as long as people understand you they can't get upset.

Just my two cents.

Let me give you an answer from the other side of the fence ;-)

We often employ freelancers using oDesk, Elance, Rentacoder and can say that we have learned to ignore generic responses for job ads, and can spot cross posters.

As somebody who hire freelancers all the time, I would say the best way to optimise the chances of success is to respond to each ad on its merits, engage with the company by asking relevant questions, the buyer is always a little unsure about the type of response he will get and is almost always overwhelmed with responses from people saying "Yes, I can do this, Please trust me".

I'd estimate that I have outsourced >$100k in the last 2 years, and every dollar of that has gone to people who took the time to engage with me through the job advert, we are all people at the end of the day, and we like to communicate!

All the best!

Floyd.