Network. If you want to build up a portfolio a good way is to volunteer to build or fix things for local non-profits, in exchange for them to give you a testimonial and let you use their name for promotion (if you do a good job, of course).
Don't compete on price. Pick your price and stick to it.
If you specialize in a particular piece of technology, spend time online helping people fixing things, writing bug reports and patches for it, creating a site with useful tools or tutorials and things like that. Then just include a small mention on your site or your signature about your freelance work.
Don't go overboard with self-promoting, but don't neglect it either.
Keep a resume (or several targeted ones if you have different skillsets) up to date and ready to send people.