ase,
You will notice that many of my clients give me sitewide links site as the one at
www.arizonatools.com which has a PR 7 and 22,000 pages. Down at the bottom of every one of those pages it says "Search Engine Optimization by SEO Guy" why? Because I did a good job on their site and they appreciate it. I get tens of thousands of links this way, it is never mandatory but when you do a good job for someone they will usually be happy to help promote your service.
I also advertise is select places like
www.searchenginewatch.com you will see my banner at the bottom left in a rotation, thats thousands more links right there, mind you if it isnt relevant dont bother a link there costs $2200 per month and they dont sell it for PR its advertising.
Also I build tons of free tools that people link to all the time, write tutorials and basically just give away as much free info as I can.
Finally my forum participation is very high at places like
www.seochat.com I've been moderating there for over a year so I get thousands of backlinks from archived posts from links in my sig.
My links didnt happen overnight and really the best advise is to provide a kickass website and provide great service and people will honestly be happy link to you, thats the sercret behind the PR algorithm in the first place, its sniffs out PR schemes etc and makes you weight (Read up on sandboxing) but if your a decent site thats been around for a while you will get your just reward.
There is also the buying route which I am not aversed to in fact I have bought a link or 2 in my day if it was relevant which IMO there is nothing wrong with **Google Please dont ban me if Im wrong** but paying $100-$500 for PR 7 sitewide $300-$5000 for sitewide PR 8 can get pricey so in the long run isnt a good strategy.