I think it's wise to take a friend out with you, that way there's someone there if you fall - to call ambluances/check you're ok. Also if the police bowl up you've got two peoples word on what happens. You could try to get a friend out with the promise of a beer after he's spotted you for a bit, who knows maybe they'll give it a shot?

yeah, i can't buy people beer yet, but oh well. I understand the benefits of going with a friend, but am only talking about when i can't get someone to climb with.

I guess my advice would be to go slow, and stay low. I don't builder that much, but when i'm out training parkour alone i'll  stay away from particularly risky things and train basics/strength - maybe that's something to do when you can't get anyone out?

A lot of people builder alone. It can be hard to convince others to come climb buildings with you (especially if there is perfectly good rock, or gyms around).

Just keep within your comfort zone. If you are thinking you might need a spot or a belay on something, then wait until you have said things.

If you are planning on doing a climb that is unprotectable with a death fall, then you are better off climbing alone anyway. It's less traumatic for onlookers should something go wrong.

I builder and boulder alone often. Once you get used to falling without spotters and climbing without people telling you how good you look it's pretty enjoyable. I always make sure I have a cell phone in my pocket or within easy reach though just in case the worst were to happen and I couldn't limp out of it. The added adrenaline rush is always fun too. Especially when holds break in the "oh shit this is going to hurt but I'll walk away" zone.