Working freelance/self-employed in Spain?

I can google this until the cows come home, but lie a lot of things in Spain, i imagine this is different per area/townhall.

Just want any general advice, anecdotes, tips, pitfalls, horror stories etc about working freelance/self-employed in Spain.

In the UK, I have a main day job, managing community care for social services. But I am also a freelance artist, designer, 3D modeller/designer/renderer, game designer. I make good money on and off, good enough to make some beer money at very least once we move to Spain. But want your real-life experiences and advice. PM or post answers.