@graeme_p How good are you with writing? If you don't particulary enjoy it, hire a good writer and go down the guest posting route. Just make sure you target bloggers that cater for the same audience that you want to target... and obviously, don't buy links, if possible :)
There are still many big sites around that accept guest posts and it comes down to what martinibuster and Sgt_Kickaxe are talking about - building relationships. Are you on Twitter? I've found that I've generated my best guest-blogging gigs via social media.
It doesn't take a huge budget to become a good guest writer, however, as with any other link building method, you don't want to solely concentrate on just one approach. I've just mentioned this as an example of low-budget link building.
Funny this, you'll eventually find that the most effective link building methods involve very little money (but a lot of time and effort)... on the other end of the scale there are expensive (and dodgy) link building methods.
If your site has i.e. 2,000 useful content pages, you can gain tremendously by posting on your own blog short posts with useful related news and information (not automatic feeds), and including links to your existing content pages.
@JohnRoy, can you elaborate on this? What do you mean?