DEAR CRABBIT: what if I can't afford professional editing?

From a blog comment:

Dear Crabbit,
It would be great if you did a post on the best things we can do if we can't afford a full-editing service, Nicola. I'm on a strict family income and spending on my writing just can't take centre stage. I can save and build some money to use, and would love to know the best way to spend it to get the best advice for my work. I can see how important outside proof reading and editing can be, so I want to get it right with my limited resources!