A lot of SEO organisations will claim to do this.

Question them carefully at to what exactly they will do - would be my advice.

I show my prospective clients my past work of keywords and websites I was successful in promoting. That usually gets my foot in the door.

Good question - the 'link building' industry is one that's shrouded in secrecy, and filled with missed expectations.

If you interview any companies, ask them to show you a few projects they've worked on, and the links they've developed for those sites.

At least, in this way, you can see if your idea of a good link matches what the link building company thinks is a good link.