The legendary investor Warren Buffett has a set of rules that he applies to all investments. I'll confess now that I haven't read his book and, to be honest, I haven't even read an article on these rules. However, I am told that one of them is to always reinvest some of your profit.
I'm not keen on hard and fast rules but I do think this has merit as a general practice. Whatever you're doing carve the income up so that you have a percentage for savings (none of my business) and reinvestment in marketing for the future (my business).
There's a lot of misunderstanding around buying services on the internet. Buying bulk links, link farms, blah blah you know all about that. However, there are still some worthwhile places to get links. Some smaller, hand crafted specialist directories are marginally worthwhile and, if you're reinvesting, then getting into one a month at £20 or so will eventually add up. This article listing 'essential' directories is worth a look, but I wouldn't take it at face value. Go to the category that suits you, find a similar business to yours and take a look at their links using a decent tool to see if it's worth it.
None of these directories on its own is going to make a huge difference. My suggestion is to invest regularly and it will become another string to your presence.
Trade organisations are nearly always worth joining if they link from their website to their members. These are usually a more significant investment of typically a few hundred pounds. You may want to make this sort of investment when you get a really good bit of business in. Again, do your research to choose which organisation. Go for one in your country and it still seems that .orgs and .org.uk have a noticable advantage.
Keep a list of places where you can invest in (rather than buy) a link, where the association will rub off on your site. The point I'm making is about reinvesting in your business marketing and how this can eventually add up to real SEO power. As I always say there are no silver bullets, but small things do add up. That summarises the realist's approach to linking really.